If you don't end up with a trade here, your options include:

Blockchain.info has added the ability to use Barclay's Pingit to buy bitcoins using GBP:
 - https://blockchain.info/wallet/deposit-pingit


Bitcoin-Central has launched its BTC/GBP market, where you can trade after transferring pounds sterling to their bank in France.  The order book is pretty thin yet but they did just launch:
 - https://bitcoin-central.net/order_book?currency=GBP


You can also send cash (GBP) in the mail to Bitcoin Nordic (in Denmark):
 - http://www.BitcoinNordic.com


And, or course, there may be a chance for a local trade:
 - http://www.LocalBitcoins.com