Most, if not all, other banks charge a fee if you withdraw money abroad. Typically, it's around 1 - 1.5% of the withdrawal, or a minimum charge of a couple of quid or so. Probably still works out no more expensive than taking a hit on the exchange rate when you buy or change travellers cheques and pay commission on top of that when you change them.
I think the Post Office Travellers card is a cross between a travellers cheque, and a bank card. You still have to pay for them upfront, but you can use them in ATMs and at outlets that accept Visa cards. I can't see any benefit in using one of these unless you don't already have a bank account with a card.