I am with Nationwide, and whenever I go abroad I just use my debit card - I don't take any other currency with me. I have used my Nationwide debit card in France, USA, Spain and Tunisia. You get the exchange rate on the day of the transaction, and they don't charge you commission. It is the main reason why I am still with Nationwide at the moment.
You are also able to use the cards in the supermarket using chip and pin - the only place we couldn't seem to use it was in the petrol pumps.
My parents live in France, and although they do have a French bank account, they also use their Nationwide account out there.
Hope this helps
Claire