I have done it regularly and had no problems at Heathrow. The last time, however, getting on the United flight in SF to LHR was a nightmare - the United people had no idea how to deal with it, were very hostile and delayed the full 747 for over an hour while they slowly realized that I knew the rules and they didn't. No idea why they could not check their own website and learn the procedure in a few seconds - took them four hours.