I've never used a foreign card in a UK ATM (as I'm UK resident and don't own such a thing).

However, agree with the above that if there's an offer of the ATM doing the currency conversion you should always reject that. This is something I pay attention to when I travel overseas to make sure I don't incur unnecessary fees. I can't remember the exact wording but it's always worth taking your time to read each screen to make sure you've selected the right option.