Gardyloo, apologies for leaving out a few details. The flights I am considering ex-SFO are on AA with a connection in DFW for MAD or JFK for LHR. I did see flights from LGW-VCE but I am not planning on changing airports. So far one friend has told us to avoid MAD because the airport can be difficult according to her.
wingzing, thanks for the info on the terminal change at LHR.