US travelers: think ahead about baggage screening at Heathrow. We didn't (our fault) and had the hassle as a result.
Of course, TSA Pre-check means nothing. They want pastes (like toothpaste) in your little liquids baggy, too, if over 100 ml. Do you know how much 100 ml is? Me either. Take out laptops and tablets. (We had three, all at the bottom of a carry-on, of course.) Try to wear shoes with no metal unless you enjoy taking them off and walking around on filthy airport floors barefoot. Heathrow doesn't really feature security on their web site, but just Google it for details.
The Brits have to be serious about security, and they are. (Did you know they removed all trash bins from the tube, because terrorists put bombs in them?) My fault for not thinking ahead on screening. But now you can do better.