M+S give a good rate, but I'm not sure if you need to have an m+s card to get it.
The Post office is commission free, and the euro's 1.19, and euro t/cs's were 1.21 today.
The Post Office credit card offers no foreign exchange fees, which means you'll get a much better exchange rate. (If you use ANY card in an ATM you'll get hit with a charge - prob �2, which can mount up over a holiday).
The rate using the PO credit card in shops/restaurants works out around 2% better than the usual exchange rate.