Travellers cheques can be exchanged at most places in SS, all charge 2% commission but still a bit better rate than the UK.
The use of debit cards are best avoided as there is nearly always a "loading charge" imposed by your UK bank.
The following credit cards are the best as none of them impose "loading charges" if you pay off your balance in full to avoid any interest. Halifax “Reward Clarity”, Nationwide, Post Office, Saga, Santander Zero. Use only as payment and not for drawing money via a cash-point, if you do loading charges will apply.
On my last visit to SS in early July 2010 my Nationwide credit card gave me 1.21 to 1.22 Euros to the GBP, much better than the 1.17 Euros to the GBP exchanged in SS.