There are 2 exchanges available for BTC/GBP, they're  https://www.gdax.com/ and https://coinsbank.com/ . But I never using these exchange before so If I were you, I wouldn't use these exchange and I'll find out another way to convert my BTC to GBP. For me, I'll convert BTC to USD first then I'll convert USD to GBP after that, although with this way you'll have to pay exchange fee twice but it's much better than use an exchange that you don't really know about it.