As I understand it you need to make a SEPA payment.

When you withdraw $ it goes to a European Bank who change it to EUROs at the market rate and it is sent to your bank as EUROs and they again change it to £s at the market rate.  It is pretty quick and painless but some UK banks (e.g. NAT WEST) stick an extra fee on top for the EURO exchange - greedy bastards!

What happens going the other way I don't know but I suspect you can't send £s but have to buy EUROs and send them to Bitstamp's Bank, they convert them to $s free of charge.  They don't mention a commission.  I have not attempted this but check with your bank on how to make a SEPA payment.

This is probably the cheapest way, any other sort of international bank transfers are a rip off.

You are, however, subject to normal currency exchange rates.