The time is 7.30pm on a Wednesday, and the place is Shepherd's Bush, west London. All along the Uxbridge Road, small, overlit counters-in-a-cupboard offer you takeaway evening meals. If you're on the lookout for gristle on a stick, or deep-fried nearly-meat and soggy chips, it's your lucky night. If not, keep walking west until you see the friendly red logo, the neon Portuguese rooster of Nando's.

Inside, you'll see that the restaurant is busy, but the waitress finds you a spot. Who else is eating? On a large table at the back is a sprawling collection of friends: some baseball-hatted teenagers, some mums and dads. Thirtysomethings Duran and Yvonne are among them. Duran likes Nando's hot chicken because "it reminds me of Caribbean food". Yvonne approves of the low prices, loves the corn on the cob and likes her chicken medium, not too spicy. "And you get your food quick!"

Nearby sits Sandro, 22, the cashier, who has just clocked off. He's from Portugal, is studying in England and has been working here for a year and a half. Now he trains new staff. "The standards are very important – customer service, the cooking and serving. I like working with the customers, even when it's very busy, like when there is football."

Right by the window sit students Asha and Faisa, in their early 20s, their beautifully made-up eyes set off by their hijabs. This is their first time in Nando's and they're impressed: "I had the chicken burger," says Asha, "it was very flavoured, not like the usual plain chicken burger. I'm going to bring my daughter here." Faisa likes the background music and how "they make it warm and friendly, it's not X-rated for kids and it's affordable for families. I don't want to leave!"

I go right around the restaurant, but no one has any complaints. And the food is making me hungry. The photographer and I go to the counter to order. There are two community police officers there, who tell me they often pick up a Nando's takeaway when they work. "The chicken is healthier than all that fried stuff. You don't get grease on your uniform."

Do you go to Nando's? Of course you do. Only those who never let a non-organic French fry pass their lips or who live a long way from a town or are hardcore multinational refuseniks don't go to Nando's. Without fanfare or advertising, Nando's has woven itself into the fabric of UK society over the past few years, popping up on high streets across the country to serve everyone from happy families to lunching workers, from gaggling teens to dating couples. Even the locally minded who campaign against Nando's arrival (London's Stoke Newingtonians comes to mind) seem to go quiet once it's established:it's a safe place for teenagers to go, and they want to hang out there.

The under-21s are not often welcomed into restaurants and invited to stay. Nando's, however, is relaxed about its teen-appeal and even encourages it, by offering free refills of fizzy drinks. And it's this, as well as its food, that has led to young celebrities, such as pop stars Chipmunk, Tinchy Stryder, JLS, N-Dubz and Pixie Lott, being spotted in Nando's. They don't mind: they all love Nando's and are happy to say so – as are other, more stately celebs, including David Beckham, Mary J Blige, Oprah Winfrey, Ricky Gervais and Jay-Z (who asked for Nando's chicken on his rider at the Brit awards).

So many of the UK's young and famous are pro-Nando's that the pap snap of a celeb clutching its red-lettered take-out bag has become a tabloid staple. The brand is now inextricably linked with a certain type of youthful, racially mixed, urban British pop star and, so, with modern, inner-city, multicultural Britain. It even tastes new. Far from the tasteless stodge of traditional UK food, American bland-burgers or no-offence Italian pasta and pizza, Nando's food is spicy, with roots in Africa and, therefore, the West Indies. "How would I persuade your mum to go Nando's?" says 19-year-old rapper Chipmunk, real name Jahmaal Fyffe. "I'd just tell her: good chicken. It's hot. It's got flavour."

Nando's peri-peri chicken and chips with macho peas and coleslaw. Photograph: Romas Foord

Chicken is what Nando's does. Decent chicken – not organic, but not frozen, nor reconstituted, battery-farmed or pumped full of chemicals either – cooked in front of you, as quick as you like. Chicken that comes extra hot, hot, medium or lemon and herb (also mango and lime, or plain, but everyone forgets about those). Each piece, even the plain, is marinated in spicy peri-peri sauce for 24 hours before it's grilled, the sauce made from a recipe based on the Portuguese Galinha à Africana method of barbecuing chicken. You can have your chicken as a quarter, half, whole or wings, with one or two regular sides, such as corn on the cob and coleslaw, or a fino side (ratatouille – ugh!). If you want chips, you can choose between regular and the very popular hot chips, sizzled with peri-peri. Add spicy or garlic sauce as you wish. And that's it. There are other ways to eat your bird – in a pitta, as a burger, in a wrap, as a salad – but most people plump for the straightforward barbecue-plus-sides offer.

Enough chicken for you? Well, no, actually. It seems that, as a nation, we want more and more. In the past few months, Nando's has taken advantage of the cheaper property market and expanded aggressively in London, opening in Westfield shopping centre, on the South Bank, in King's Cross. Outside London, it has opened in Belfast, Thurrock and Glasgow. There are now 220 Nando's in the UK, in every large town from Edinburgh to Exeter, serving 800,000 customers every week. Even if some of those customers visit every day, and some just opt for a salad, that is a lot of Nandinos – and a lot of chicken.

Nando's is one of a few modern restaurant brands to have changed the face of British fast food. In recent years the Gourmet Burger Kitchen, promoting its Aberdeen Angus beef and use of Peter Gordon, the father of fusion cuisine, as a consultant, and Wagamama, with its fresh, fast pan-Asian food, are others. All promote just one part of that "fast" deal, in that they serve you fast, but they don't push you out the door at the same speed. No one is saying they're a foodie's paradise but they all offer healthier options than the usual take-out joints. As Jamie Oliver pointed out recently, these days the UK is far less tolerant of rubbishy food than the US. Even McDonald's in the UK has busily revamped its offer – its eggs are free-range, it uses organic milk and its adverts constantly remind you that carrot sticks are on offer (one of your five-a-day, kids!).

So is that Nando's secret? That it isn't your usual takeaway joint? Does that explain its popularity? Ask the PR team – which consists of just two people, one of whom is on maternity leave – and you'll be guided to the chain's "family feel". This is Nando's professed USP. Yes, it prides itself on its spicy chicken, but it's the friendly atmosphere, both for workers and clientele, that keeps its fans loyal. Customers know what they're getting, which is: quick, smiley service, clean surfaces, big tables you can spread out over, South African art on the wall (Nando's commissions original works). There is always lively African or Latino music playing, which helps cover your embarrassment if you have noisy kids. Yet, each Nando's is very different architecturally, adding to that non-standardised, proper restaurant feel. The South Bank branch has high, arched ceilings; King's Cross has an outdoor space; Brixton is wide and low.

The way customers are encouraged to act is different, too. There's an element of responsibility, in that you have to go to the counter to order (although the waiting staff bring your food over and everyone on your table is served at the same time), and you help yourself to your own cutlery, fizzy drinks and sauces. All this moving about helps create a fluid, busy atmosphere – but you can stay as long as you like.

As for the staff, the PR assures me that Nando's is keen to offer its workers opportunities to advance themselves, many grillers and waiters moving up the ranks to managerial status. There are away days and other social events, and the company won the Sunday Times Best Place to Work (Big Companies) category this year, with 75% of the human resources budget invested in learning and development. Staff are young (65% under 25 in 2005), but staff turnover is low.

All too good to be true? Maybe. Though it opened in the UK way back in 1992, Nando's still feels like the new kid on the block. It has taken its time to make an impression and we're still excited by it, not bored yet. Still it's no stranger to bad press: the most serious being when one customer was shot by another, in this very Shepherd's Bush branch, in 2002. The victim subsequently died.

In Shepherd's Bush, at the counter, I ask the community officers about the shooting. One shrugs. "It's a terrible thing," he says, "but it's not Nando's fault. It could have been anywhere. It's not like the guy was shot for his chicken, was he?"

Nando's origins lie much further afield than west London: not in Portugal, but in South Africa. In 1987, Robert Brozin and his friend Fernando Duarte popped into a Johannesburg restaurant called Chickenland. The chicken was the best they'd ever tasted, they thought, and promptly bought the joint. They renamed it Nando's, after Fernando, a Portuguese national.

Despite its hoo-ha about happy staff and customer service, it's Nando's peri-peri sauce that is its basic selling point. It's a version of the piri-piri marinade used on chicken in Portuguese restaurants the world over, which was created by Portuguese settlers in Africa: they used local chillis (Nando's favours chillis from Mozambique), known as pili-pili in Swahili. Now you can buy Nando's hot sauces and marinades in supermarkets for around £1.60, as well as Nando's crisps. One happy online user described the marinades thus: "Turns ordinary, boring, everyday chicken into a Festival of Chicken, complete with chicken-skin streamers and party giblets". Mmm.

In Nando's in Soho, at lunchtime, I meet Matt, 39, and Bryan, 24. They work in documentary photography and have been to Nando's in Africa. What's the difference? Bryan thinks that the London Nando's are "a little louder, bigger and more open. And a little more uppity [upmarket]." But, really, says Matt, "it's pretty much the same. You can count on it being consistently good."

Differences do exist. In South Africa, Nando's is seen as more of a takeaway; it wasn't until Nando's in the UK began to emphasise its restaurants over its take-outs that the brand really took off here. Now around 80% of its food is eaten in. Also, in South Africa, as in Australia, another huge Nando's market, Nando's is marketed as a jokey brand. It does a lot of TV advertising, and its ads are often reprimanded as being in poor taste: a 2009 campaign featuring a Spitting Image-style puppet of Julius Malema, president of the ANC youth league, was removed at the behest of Malema's lawyers. Actually, to my eyes, the ads are dodgy, with black Africans often represented as being stupid or naive.

In contrast, in the UK, Nando's has almost no advertising presence at all. I ask Malcolm Pinkerton, a retail analyst at Datamonitor Group, why it hasn't bothered. "Prime locations appeal to people without having to advertise," he says. "Then, if you have a good product, you can rely on word of mouth and social media." Nando's has a strong Facebook presence: there are campaigns for Nando's to open in their particular area (Wigan, Peterborough, Northwich, Burton-on-Trent) and former workers have groups, too.

What's unique about Nando's in the UK, however, is its status with the city youth. Over the past few years, the UK charts have been transformed by British-born urban pop artists, such as Chipmunk, Tinchy Stryder, Dizzee Rascal, N-Dubz, JLS, Taio Cruz… and every one of these No 1 artists is a Nando's lover. They would be, even if they weren't successful. They are typical Nando's customers, with parents or grandparents born outside the UK who have brought up their offspring to have a spicier palate. Social and confident, happy to hang out with mates, these kids are easy with eating out and expect to be treated well when they do. Nando's ticks all their boxes. Now, its chicken is the urban youth food of choice and, as if to prove it, I have two quite intense phone conversations about Nando's, with Chipmunk and then Tulisa from N-Dubz.

Chipmunk, born in Edmonton, north London and named best hip-hop artist at last year's Mobo awards, tells me he often eats at his local branch of Nando's in Finsbury Park. He favours the wings, medium spicy: "It's the closest thing you can get to jerk chicken, and if chicken isn't Caribbean or Nando's, I can't mess with it." He likes the atmosphere, how the staff are always smiling, and that "you can eat your chicken with your hands. It's not too booji [bourgeois]. Even though you do get posh people in Nando's, it doesn't outlaw any social class."

While researching this piece, I'd heard stories about a Nando's "gold card", awarded to a very small number of extra-special customers and entitling them to free meals. This mythical piece of plastic is so valued, so sought after that, initially, Nando's PR would not confirm its actual existence. It turned out they've been having problems with people faking the cards, or passing the cards between each other.

Chipmunk, though, along with Tinchy Stryder, is one of the privileged few. "Yeah, I got one!" he says with glee. "It's not gold, it's black with gold writing, and it means I can get food for free for me and four friends. I don't pay 'cos I love Nando's and I promote Nando's and I'm happy to do it."

Tulisa Contostavlos, 21, Camden Town-born actress and singer with double-platinum hip-hop influenced pop group N-Dubz, however, does not have a card. Which seems ridiculous, given just how often she eats there. I interview her in the middle of the N-Dubz national tour and she appears to spend most of her off-stage time in Nando's.

"When I wake up, say around 1 o'clock, the first thing I eat is Nando's lemon and herb chicken in pitta," she tells me. "I don't like cold food in the morning: I'd rather warm up last night's curry and have it for breakfast. I like the texture of the chicken, it's not too crispy, not too soggy. Anyway, I put some peri-peri sauce in there and I might pop in a few chips. And then, when dinner time comes, I have a quarter lemon and herb chicken with spicy rice. And chips – normal, not spicy. You have to get ketchup – one teaspoon per chip. I drink Sprite with it. Even when we're not on tour I eat there at least once a week. I should have my picture on the wall."

She is very jealous of Chipmunk's Nando's card and, in fact, this causes some problems: after the interview, we ask if we can set up a photograph and N-Dubz's people say: "Only if the band can get Nando's cards." But Nando's PR denies such cards even exist. I point out that Chipmunk has described his to me and Tulisa has seen it, and she then counters that there's a waiting list. Hmm. No card, no photograph. Such is the value of this particular piece of plastic. And we are asked not to photograph Chipmunk's card – to stop the fakes out there.

It's interesting that so many black and mixed-race kids are into Nando's: that combination of spicy, almost jerk, chicken and the space and respect each customer is given is a winning formula for the kind of people that most restaurants choose to ignore. When I talk to customers in Shepherd's Bush, Soho and Brixton, the only other plus that comes up a lot is health. The chicken in Nando's is deemed by its fans to be better for you than anything from your standard fast food restaurant. But is it? It's not battery-farmed, but it's not free-range either. The suppliers that Nando's use – Faccenda and Sun Valley for everyday chicken, halal from Freemans of Newent and Highbury Poultry – comply with the voluntary, industry-led assured chicken protection standards. Nando's also gets some extra chicken breasts from suppliers that meet similar standards in Holland and Germany. Essentially, the chickens are reared in well-littered and ventilated houses and have constant access to water and food that isn't spiked with growth promoters.

Nando's say that their chicken is in constant review and they are actively looking at alternative methods of production, including RSPCA Freedom Foods. The problem is price: the amount of chicken Nando's needs and the price it charges (£9.10 for half a chicken with two regular sides) means that, for the moment, organic just isn't an option.

I think of Nando's with more affection than I do of, say, Starbucks: if I'm in an unfamiliar city with my son and I see Nando's, I'm pleased – we can both eat food we like, relatively cheaply and definitely cheerfully. But if I have a choice between Nando's and my local Portuguese restaurant, there's no contest: Brazas will always win out. Nando's is a nice experience, but it's a standardised one, and my tolerance for same-same is lower than Tulisa's.

What of the future? Capricorn Ventures, which owns Nando's in the UK, is promising to expand even further, declaring that Britain can support up to 400 branches. Two Nando's have opened in Washington DC, to great excitement. Robert Pattinson is rumoured to be beefing himself up for his next Twilight film by eating Nando's. It's a chicken-fuelled world out there, people.

In Nando's in Brixton, Robert and Jameela are having an early evening meal. Robert is a regular. "I like my chicken twice a week. Always medium, with peri-peri chips and coleslaw on the side." Jameela: "I don't come as much as Robert, but I look forward to it when I do. I love Nando's! Why? No special reason. Just that the chicken tastes so good."