It's funny but the USA has been a bit behind the times with credit cards. It was only about a year or so ago I actually had my old magnetic strip only card replaced with a chip card. Going back about 3-4 years ago I tried to pay for my brother's petrol at a filling station and was told since I didn't have a chip card they couldn't accept it! Interestingly last year I "paid" with my credit card at a small UK Indian restaurant and the charge didn't appear, contacted the owner and he didn't get payment -I was back in the States by then (did get my sister to drop by and pay the bill in cash however). Don't know if the owner made a mistake, but my credit card company didn't seem to have their act together just telling me the restaurant charge was subsequently "removed" for absolutely no reason -that owner could have been easily stiffed.
So given all that, I think it's safe to say I won't use a contactless US card even if I had one unless I'm at the point of sale with a real person (BTW, are contactless cards the same as chip cards?).
Either way, I'll have time on my side to get transportation sorted out, so that won't be a problem, and I'll be at LHR to do that with no great hurry. And... DC/CR = Docklands? What does CR stand for? Guessing not Costa Rica. Just tried to google with no luck. If I'm at LHR I'm guessing it's still easier to get a touristy oyster card. Thanks again for all the responses.