best way is via transfer wire to bitstamp...
there is an issue atm accepting gbp at exchanges, so most wont accept...
I use transfer wire -> euro -> bitstamp
takes about 48hrs, and costs very little due to conversion via euro
See my sig for a free transfer from transferwire
although bitstamp is offline atm (has been for an hour two), it at least is a uk company
he wants to
sell bitcoins for GBP, not buy them
localbitcoins is an option, but it can be a bit of a hassle sometimes
using an international exchange exposes you to transfer fees
i would say your best bet is
bitcoinfridge.co.uk