http://www.theguardian.com/media/2014/mar/18/lenny-henry-black-asian-television
Lenny undoubtedly (and yet again) makes a valid point.
Talking comedy specifically (though it often can apply to drama), the perennial problem is, as I see it, that black and Asian actors/characters are far too often only really of prominence in 'ethnic' shows. If an actor is black/Asian/other minority, it seems OK to cast them in an 'ethnic' show - such as Citizen Khan, but in a 'white' show, non-white actors are rarely cast solely on their ability.
Why shouldn't a black/Asian/other actor just be cast on the basis of their suitability for a role? Maybe the best example of this in recent years is Paterson Joseph's brilliant performance as Johnson in Peep Show. This genius of this character has nothing to do with the fact that he is portrayed by a black actor.
This should be the way forward really. Most deliberately 'ethnic' sitcoms - although rare - often seem handicapped by being written by and for an ethnic audience. Of course, there are loads and loads of bad/mediocre 'all white' sitcoms.
The way forward lies not in giving non-whites more of their own 'token' sitcoms, but to get producers/commissioners to not be so afraid of casting non-whites in sitcoms where the actor's ethnicity is not of major relevancy to the character they're playing. Some of this responsibility lies with writers, of course, specifically not writing 'token' non-white characters and then loading those characters with stereotypical ethnic traits/concerns.
It seems rather weird that this situation still exists in 21st Century British TV, but unfortunately it does.