t'internet suggests that the EUR ATM at Nat West on Bishopsgate (back side of the old Nat West Tower) does work on a native basis so will actually charge the card in Euros.

Thing is, if you're in Kingston, it's a bit of a schlep up to the City to do this when pre-booking will get you a decent rate at the airport anyway. You'd probably need to take £2k out to cover the cost of getting there & back.

http://forums.moneysavingexpert.com/....php?t=3739431 is interesting but may be out of date.