Peter Cuthbertson has contributed an interesting post CentreRight: After Obama, could it happen here? Yes, in 1868 noting that the UK had a Prime Minister from an ethnic minority 140 years ago. But many people now no longer list Jews amongst ethnic minorities and so forget Benjamin Disraeli.
I also remember a moment of the Question Time US election special when someone in the audience said that the United States is the only place where someone raised by a single mother could become leader. Again the UK has been there long ago, when in 1924 Ramsay MacDonald became Prime Minister.
So how long will it be before we see a non-white Prime Minister? It's hard to say because leaders rarely suddenly emerge in British politics but rather spend some time in the Commons first, and to describe someone as "going to be Britain's first [something] Prime Minister" is a kiss of death to a career. But I think after the next election we will be seeing several potentials on the front benches. This country is used to being led by non-white Britons - look at sport! All we're awaiting is the right candidate in politics.
(P.S. I've been told that by the US definition of these things we've already had a black Doctor Who. Can you guess who? And no, it's not Lenny Henry.)