Lenny Henry isn't laughing. I've told him a joke but he doesn't crack a smile. Instead, he wearily tells me that it's too old, and not funny any more. Perhaps it's not surprising. Now a critically acclaimed actor, Henry is almost unrecognisable as the cuddly comic whose suits were once as loud as his bellowed catchphrases. Lean and aloof, today he remains straight-faced as I repeat a quip told by comedians from Gina Yashere to Stephen K Amos – that the only way for a black comedian to get their own TV show, is "to wait for Lenny Henry to die".

"I guess when I had the Lenny Henry Show that was a decent joke to make," he says grudgingly, before drawling, "But I'm doing theatre now darling, so it's time for you to do your show."

I'm surprised by his sensitivity, because the gag's point is the absence of other ethnic minority stars on our TV screens – something the 55-year-old has been attacking for years.

In March Henry hit the headlines with a witheringly funny speech at the British Academy of Film and Television Arts (Bafta), taking the sector to task for its "appalling" record on diversity. He exposed the uncomfortable truth that not only was the television industry nowhere near as diverse as the population as a whole, but that the number of black, asian and minority ethnic (BAME) workers was plummeting.

"From making very small gains in the industry, when we had a little bit of representation, we have gone backwards," he tells me. "People aren't just hitting the glass ceiling: they are standing on a glass precipice."

The statistics speak for themselves. While the UK population, he said, is 14.5% BAME (rising to 40% in London, where most of the sector is based), currently only 5.4% of the creative industries' workforce is non-white – the lowest figure since the monitoring group Creative Skillset started counting. "Between 2006 and 2012, the number of BAMEs working in the UK TV industry has declined by 30.9%," he pointed out, going on to say that in the last three years, "the total number of BAME people in the industry has fallen by 2,000 while the industry as a whole has grown by over 4,000. Or to put it another way – for every black and Asian person who lost their job, more than two white people were employed."

The haemorrhaging of black and Asian actors to the US – from Archie Panjabi and Chiwetel Ejiofor to David Oyelowo – is just the tip of the iceberg.

It's a subject Henry is uniquely qualified to broach. It is almost 40 years since he first appeared on British screens, as a 16-year-old from Dudley in bow tie and flares who did celebrity impressions – "in colour" as he cringingely put it at the time. He went on to become the best known black British entertainer on mainstream TV – and for too long, the only one.

It's a position that is at once a privilege and a problem: he has been a pioneer, and a lightning rod for prejudice.

While still a teenager, he honed his act in working men's club of the 1970s, where the racism could be brutal. "I was literally sat in a toilet in a cubicle," he tells me, "and heard someone say, 'What's this darkie like then? He better be funny or we'll string him up in the car park.'"

His routines back then, if not in tune with these attitudes, hardly challenged them. His act included jokes about his sweat tasting like chocolate, and he even – to his regret – accepted a part in the Black and White Minstrel show. It was only when he was cast in the country's first all-black sitcom, The Fosters, a few years later, and was taken to task by his co-star Norman Beaton, that he began reassessing his act.

From then on he tried to ensure he was playing "archetypes not stereotypes", he explains. By the time alternative comedy became popular, he was an enthusiastic supporter of the scene, which refused to use the then-traditional comedy fodder of gags about women and minorities.

The reaction when he married comedian Dawn French in 1984 was proof that racism was not confined to the comedy circuit. "It was a scary time because we were a pretty high-profile mixed-race couple," he remembers. "The News of the World had put a picture of our house on their front page, and the National Front came round and smeared excrement on our front door, and tried to put petrol-soaked rags through it."

Dawn French and Lenny Henry's wedding in 1984. 'It was a scary time because we were a pretty high-profile mixed-race couple … the National Front came round and smeared excrement on our front door.' Photograph: PA

No wonder he is unimpressed by critics who suggest he is not radical or edgy enough. "I got accused of misrepresenting all people of colour in Great Britain. I would get told off a lot. 'How can you do African characters when you're not African?' But I gave it a go. Maybe if there had been more of us I could have just been Lenny Henry from the Black Country with Jamaican parents."

Through his production company, Crucial Films, he created a new writer's workshop that led to the development of the UK's first black sketch series, The Real McCoy. It was very successful, so Henry finds it perplexing that none of the black comics was given their own show, and claims it "was airbrushed out of" BBC2's recent 50th anniversary celebrations.

In the 90s he played Gareth Blackstock in Chef!, in a then-rare example of colour-blind casting. While on the Lenny Henry Show, he continued to champion comedians such as Little Miss Jocelyn and Yashere. Yashere said she had to move to the US to find the opportunities she could not find in Britain.

In 2008 he gave a speech to the Royal Television Society that was a precursor to his Bafta lecture ("It's good to be back, people" was his sarcastic opening to Bafta). But if he is fed up making the same case, it doesn't show.

"This is something worth fighting for," he tells me. "Why wouldn't you fight for this? It's not just about showbusiness – everywhere you go people are discriminated against. And if by having an organised voice against inequality and a lack of diversity we might be able to push that down – how brilliant would it be?"

To this end he has drawn up a proposal for structural change at the BBC: the Henry papers, or "I have a screen" as he has dubbed it, is modelled on the corporation's success in increasing regional diversity – ring-fenced money, specific commissioners for BAME programmes and a focus on talent behind the camera as well as on screen.

This week BBC director-general Tony Hall rejected Henry's proposal for fixed quotas of BAME staff on certain productions, but announced a new BAME development fund, new training opportunies, and welcomed Henry's acceptance of a place on a new Diversity Action Group.

Henry says: "I think it's a really positive small step forward, and it sounds like a cliche, but a journey of a thousand miles starts with a small step." He hopes other broadcasters will now follow the BBC's lead.

But he still thinks a solution more like the one adopted by the BBC in relation to the nations and regions would have been "more satisfying". The BBC's commitment to spend 50% of its budget outside London by 2016 has, he thinks, delivered huge change. "People are in denial if they think quotas don't exist," he says. "There are quotas for the amount of news, for the amount of children's TV, surely BAME people have the same rights as Peppa Pig?"

Last month a Ukip council candidate was forced to resign from the party after tweeting that Henry should emigrate. Henry says: "I was born here. I grew up in Dudley, and my first job was in British Federal welders. My mum said (he puts on a heavy Jamaican accent): 'HIN-ter-grate. If you want to get on in this country you have to HIN-ter-grate.' So I sat at my friend Gary's house eating egg and chips, having tea with his parents and watching Celebrity Squares. I'm part of this country." He was cheered by the support he received from other social media users at the time, and says such attacks will not put him off. "It is tricky, because if you put your head above the parapet, people take potshots.But I want something done now. Shakespeare said we need to hold a mirror up to this society, and we do. If you can't see, you can't be. If we don't see BAME people on the TV, or in film, we become invisible."

It's very difficult for people to understand how diverse this nation is if we keep giving [BAME] programmes one or two series and then getting rid of them."

Henry says that while the BBC is hardly the worst offender against diversity, it is held to a higher standard. "The BBC charter says it is supposed to represent the country's nations, regions and communities – and it is not fulfilling all of that promise." Whether he thinks the BBC acted correctly in not sacking Jeremy Clarkson after the Top Gear presenter was filmed using the n-word while reciting a children's nursery rhyme, he won't say.

Instead he recounts a story of his own insensitivity. "I once did a Zimbabwean comedian [character] with a grass skirt and tribal markings. A man from Nigeria who was working at LWT said to me, 'Len, are you from Africa? Do you know what Katanga means? Do you know where Katanga is? Then why are you playing a Zimbabwean character?'

"If he had been on my production team, then I wouldn't have done that character," Henry points out. "But if you have a monoculture you will get these things happening all the time."

Reading on mobile? Click here to watch Lenny Henry Bafta speech

This week Tony Hall announced 20 new traineeships for BAME graduates, but Henry says such schemes are no longer enough. "Off screen, it's about the people green-lighting [programmes], and hiring and firing. Behind the scenes, it is very male, very white. People employ who they know. They don't say, 'Let's do a diverse casting process', they just say: 'I'll ring Charlie because he was good last time.'"

He suggests a return to the ethnic minority departments that broadcasters used to have, which were closed, he claims, due to "cost-cutting". "We need them back – and not just one person looking grumpy in an office with 'Diversity' on the door."

Henry has certainly been working hard. Alongside meetings with the BBC's director-general and the heads of Channel 4 and ITV, he has spoken to Ed Vaizey and Vince Cable, and has set up a meeting with the Labour party. Meanwhile, the TV Collective, an organisation campaigning for diversity, has warned that BAME audiences may stop paying their licence fee if nothing changes.

Henry shies away from this, but says he is in favour of marches. "Maybe the next step is to be more angry. I think we have been incredibly calm, and perhaps the next step is to manifest ourself as a group and be more angry."

In the meantime, Henry has a clutch of new projects coming up, most of which he has written himself. "Writing has set me free. I was a bit trapped, feeling that I couldn't write, and I was stuck.

"Not coming from Oxbridge, I was intimidated by people who used big words and had a massive frame of reference. It was never on the cards for me to go to uni, going to a sink secondary modern in the Midlands. The threat was that if you didn't get your exams, you would end up in a factory. Which I did, but actually that was a great experience, and I would have even got an engineering diploma if I had stuck my three years out. But showbusiness came in the middle of it all, and I left skid marks outside the factory walls! I did my O levels while playing the summer season with Cannon & Ball in 1981."

Fittingly for his 40th year in TV next year, one of these will be a programme he has written, based on the first two years of his career. "They've got to find a 15-year-old black kid who can do impressions of Harold Wilson and Tommy Cooper," he deadpans. "I wish them luck."