I've got a Nationwide Select Credit Card and there's no charge - it's converted at the Visa Spot Rate.
I've also got this new Travelex Supercard which you can link to both your Credit and Debit Cards. It was great when I used it on ATMs in France in Jun/Jul -no transaction charge or conversion charge .......but it no longer works. Travelex now say the transaction must be completed within 3 seconds after the PIN is entered.It's not possible on French ATMs