Slickw, I did IAH-LHR-EDI back in April. When you are at LHR, be sure to follow FLIGHT CONNECTIONS. I was doing this and one of the Purple Coat workers saw my US Passport and motioned me to go to the Border Force line. I said I had a connection and pointed down the hall towards "Flight Connections". The response was I would have to collect my luggage an recheck being on a US Passport. I knew this was wrong but she was un-relenting. Needless to say I alerted a border force worker walking past the line after queuing for an hour. I told her what happened and she couldn't believe they did that and she escorted me on to Flight Connections. There is passport control there just for people who are on connecting flights. You will also have to go thru security again but these lines were maybe 2 minutes long. Needless to say I spent 4 extra hours at LHR, BA bought me lunch, and I still made my 3:30 PM tee time at LadyBank.