As per EUR or USD to GBP, yeah, I don't think there are any big enough BTC/GBP markets.
I've been looking at this the last few days and the cheapest solution I seemed to have found so far is:
1) trade out of BTC into EUR on Kraken for 0.2%
2) SEPA transfer to a EUR bank account in the UK
3) Use a cheap FX company to change into GBP (found these guys earlier: http://www.worldfirst.com/) and doesn't charge for transfers.

Sounds good no?