Hello Smurf - well, we never check luggage, and it's my understanding that one can bring two pieces of carry-on on flights from the US to Heathrow, but only one on flights leaving Heathrow. That's why I'm thinking I need something that I can fold up and put into my larger bag on my way home. (I plan on bringing a coat with lots of big pockets as well and loading up the coat pockets with items from the smaller bag until I get through security.)

If anyone else has any additional wisdom on this subject, I'd love to hear it.