Several UK exchanges will take euro coins, but at a horrible rate like 3 to 1, so you might as well just keep them until the next time you visit. There is/was a machine at LGW that converted €0.05 into £0.01 (but occasionally crappy European vending machines will convert 1p into 5c - I once tried to get rid of some 5c but pulled the coin return lever and got a bunch of pennies back )

Someone in the opposite situation to you could sell the coins on ebay, as they seem to be going for more than face value on the UK site but not the US site...

I have exchanged coins with the people sat next to me on the flight several times, but that involves being lucky and brave enough to strike up a conversation!