Depends really...where I used to live, around 40% of the population were black/Asian, so when they turned on the telly it looked completely normal
However, if you lived in a small town (say) in Hertfordshire and never saw a black/Asian face, I think it helps having different racial groups on telly because otherwise you could end up ignorant of 'who they are'.
Also, 'over-representation' in some fields is balanced out by under-represntation in others - you could watch the UK History, Discovery or Bigography channels for a week without seeing a black face.
I've just checked out the official UK singles chart for the week ending 31st March. There are six black acts - 15% of the chart...and bizarrely the same bloke (Akon) turns up on three different singles.
Rap/R&B isn't as popular as you may thing. The UK is currently going through an indie rock revival (Kaiser Cheifs, Fratellis, Artic Monkeys etc etc).