Have been looking around online for the best deal on sterling travellers cheques and came across Travelex.co.uk, (am not sure if this is only online). They seem to be doing all American Express travellers cheques including sterling commision free. They even deliver by special delivery for free if its over £500 or you can collect at one of their branches.
I think most credit cards class this as a cash advance and will charge you accordingly so watch out.
I think most debit cards should be fine though (but please check first).
Seems to good to be true. Let me know if I've overlooked some hidden charge because I can't find one.