If there’s one thing Sasha Khan loathes, it is shopping for swimwear. Which makes it a strange coincidence that, on the day we speak, Khan has just been bikini shopping. “I buy swimwear about once every 10 years,” she says. The 23-year-old charity worker from London is flying to Ibiza tomorrow. “I always put it off. I hate it. It’s like having to go to the dentist or get your eyes tested.”

The reason Khan hates bikini shopping is because she is a size 10-12, with a 32FF bust, and few retailers design swimwear for women with relatively narrow backs and larger cup sizes. Khan isn’t alone. “I heard at least two or three other people in the changing rooms today saying: ‘Oh God, I can never find anything.’”

It will be a familiar grievance to many women as holiday season approaches – frowning at their reflections in starkly lit mirrors as they attempt to wriggle and contort themselves into high-street swimming costumes that seem to be made with no consideration of women’s bodies. “It’s such an emotional drain,” Khan says. “Even if you’ve bought something, you always leave feeling deflated afterwards.”

Canvassing women about their experiences of buying swimwear on the high street, the feedback was astonishingly negative: tales of bikini bottoms so tiny that most of your rear end is on show, or cut too tight on the thigh so that you can barely get a leg inside them. Taller women find swimsuits are never long enough, so they get a perpetual wedgie. For women who don’t fit into the 6-16 range offered by most high-street shops, the choice is limited to matronly control-wear and billowing kaftans – the assumption being that plus-sized women are ashamed of their bodies and want to disappear.

Youswim costumes are made from stretchy fabric that accommodates a range of sizes

“It fits into the narrative, which I think a lot of us have internalised, that your body is wrong for the clothes, rather than the clothes being wrong for your body,” says Stephanie Boland, 28, from London, who is a bra size 28FF. “It’s depressing to buy things that are supposedly in your size and find they don’t fit at all.”

Often women end up with the thing that is the closest to fitting. Plus-size blogger Hollie Burgess, 33, from Nottingham, remembers her first holiday abroad, aged 18. All she could find in her size was a brown tankini, which she hated. “My size just wasn’t there in shops,” she says. “I remember thinking: everyone else can wear what they want and I’m wearing this.”

Things are looking up, however, and it’s all thanks to the internet. “If we think about online providers, in effect they have endless aisles, which means that they can store unlimited inventory,” explains Jonathan Reynolds, a retail expert at the University of Oxford. High-street shops tend not to stock what is known as the “‘long tail’, stuff that might broaden your range, but doesn’t sell frequently enough to justify its space on the shelf”.

This may not be a sensible commercial decision for long. According to market researchers Mintel, 18% of women buy plus-size clothing, defined as a size 18 or above. Online-only retailers such as Asos, Missguided and Boohoo have proved particularly adept in targeting these consumers. And, even for women who are not plus-sized, online shopping takes the sting out of swimwear purchasing. “No longer are people going in, picking up a swimsuit from a rack, going to a changing room, trying it on, looking at it in the mirror with harsh lighting and feeling bad about their bodies,” says Steven Wright, a senior lecturer in fashion design at the University of South Wales. “What we’re seeing is people purchasing multiple swimsuits online, trying them on in their own home and sending the remainder back.”

Bravissimo specialises in swimwear for women with larger busts

Brands catering to women shut out of the British high street have experienced remarkable growth. Bravissimo, which specialises in swimwear for women with larger busts, saw its sales increase by 3.4% to £51.9m in 2017. Lyra Swim – a modest swimwear brand founded in 2017 by Ikram Zein, who was tired of never being able to find swimwear that felt acceptable for her Islamic faith – has grown 300% year-on-year. “I’m surprised by how quickly we’ve grown,” she says. This is particularly because not all of her customers are Muslim – some want to cover up because they want sun protection or feel body-conscious. (Nigella Lawson was spotted wearing a burkini on Bondi beach in 2011; she didn’t want to get a tan.)

New players are entering the market. Elise Wallbank launched Youswim in June 2018, after discovering a stretchable nylon fabric that could be made into an outfit that would accommodate a range of sizes. The brand’s swimwear fits size 6 to 14, but Wallbank acknowledges there is “work to do in terms of being more inclusive” for bigger women. Browsing Youswim’s website, women of different sizes beam back at you. They have pot bellies, wide hips and thighs that jiggle. They look wonderful. “From the outset, I never wanted the swimwear to be displayed on your typical size 8, blond model on a beach ... because it works on such a diverse range of sizes, it seems counter-intuitive to not represent that market in the way that you advertise your product.”

Brands like Youswim have a simple aesthetic – minimal one-pieces and bikinis, without adornments, underwiring or padding, in a stretchable, one-size-fits-all “crinkle” fabric – that is indebted to luxury swimwear brand Hunza G. Even if you have never heard of Hunza G, you have almost certainly seen a knock-off by now. There are no dangling bits of string or complicated halter-tops to tie. “I didn’t want to design something that felt really complicated that you couldn’t swim in,” explains the brand’s creative director, Georgiana Huddart. “Years ago, all the brands that were really successful had cutouts, diamante bits, little metal things hanging off them. It was unusual to find something quite sporty and simple, but fashionable.”

Hunza G costumes and those like them are popular because the material moulds itself to your body, meaning it fits women who are different sizes on top and bottom. As the material is thick, you feel contained – your bits aren’t going to lurch anywhere. “Fashion has taken a turn for the functional over the last few years,” explains Wright. “What we saw 10 years ago was effectively several triangles of Lycra worn on the body. Now we’re seeing moulded, tech-fabric-driven garments that provide the body with a bit more of an equipment base.” But with designs starting at £130, Hunza G is expensive, and it is not currently suitable for plus-size women, only fitting up to a size 14. (Huddart plans to introduce a version to fit up to a size 20 next year.)

Lyra Swim is a modest beachwear brand. Photograph: WWAGS

One retailer cornering this market is Wolf & Whistle. With sizing from 32B and 42F, and a dress size 26, it offers trends-driven swimwear that caters for plus-size and big-breasted women. “Often when people do plus-size or fuller-bust swimwear, they make it dowdy,” explains the co-founder Emma Parker. She wanted to make swimwear that was as “sexy and fashionable and revealing and brightly coloured” as anything on the high street. The bigger range of sizes does add to the brand’s production expenses, but Parker thinks it is worth it to cater to women of all sizes. “It normalises bodies, having them all in there together.”

Eventually, Khan bought a yellow bikini from Bravissimo, a specialist store for women with larger breasts. It fit perfectly, but, at £60, was more expensive than she was hoping for. “The garments cost more to produce,” explains Clare Turner, a senior supply and merchandising manager at Bravissimo, “[because] if you’ve got a swimsuit on the high street that’s just a pull-on swimsuit, it could just use one fabric, or maybe a very thin liner. Whereas a cup size and wired swimsuit might have the outer fabric, the liner in the body, and then it’s essentially got a full bra inside ... it’s two products in one.”

There’s still a way to go, particularly when it comes to sustainability. Wolf & Whistle’s recycled swimwear collection isn’t available in plus-size. Although Youswim’s garments are ethically manufactured in the UK, and Lyra Swim uses a Turkish factory that is audited by the Fair Wear Foundation, it is still the case that many consumers favour fast-fashion brands. Although they may have affordable and size-inclusive ranges, some such brands have poor records when it comes to worker rights and are some of the biggest contributors to the estimated £140m of clothing sent to landfill every year.

Gripes aside, there is no doubt things are improving. “You have a choice now, whereas you never used to,” says Burgess. “Different shapes, different cuts, different colours. The brands are slowly doing better.” She tells me about her favourite yellow swimsuit. It is from Forever 21, and has a cutout detail and bow at the front. “It’s miles away from the tankini I bought when I was 18!” The first time Burgess wore it, on holiday with her fiance in Magaluf in 2014, it was game-changing. “I felt comfortable in what I was wearing, and I felt good.”

• This article was amended on 11 June 2019 to clarify that Lyra Swim uses a factory that is audited by, rather than part of, the Fair Wear Foundation.