Last year we used a post office card, put plenty of euro's on it to be going on with and took a much thinner wad of cash than usual. The card wasn't much use in Italy where almost everyone wanted cash only so we did have to visit a hole in the wall to get more cash a few times but it was just the job in France. I cocked up the sms format needed for getting the balance so for a while was less than impressed I had to guess what was left on the card and was blaming the postoffice
You can load the card from your normal bank account via a SMS message so you can load it as you go through the holiday, which saves guessing how many euros you'll need for the entire trip. It's probably no cheaper than a normal bank debit card, might even cost a few pennies more, but if your card gets stolen, cloned at an ATM, whatever, only what's in the postoffice card account is at risk not your UK bank balance. So I think we'll use the PO card again this year and just be better prepared this year that by the time we get to Italy we need more cash!