Just to give some data out on vouchers, I bought the 30 quid one today and it cost me €34.50 with Amazon converter. That works out at an exchange rate of 1.15 against the 1.11 official market rate, in increase of 4%. if you pay in GBP and use your own credit card company´s rate, you will probably pay 3% more (standard CC commission) so the difference is negligible, unless you are purchasing a huge amount or you have a no comms deal on your credit card. You will only find out the hit once the transaction is done and can´t cancel so Amazon´s converter is a safer, if more expensive method.
Applying the 7 pound promo to a 30gbp purchase gives a rate of 0.93 - a rate unlikely to be beaten unless the sky falls in on the UK.