The VS19/20 LHR-SFO-LHR are mostly served by a 744 but very occasionally by a A343 but not the A346. You can go to http://www.virginflyer.com to check your chances of which aircraft you might get on to SFO.
The flight as Sky said is a 747-400 with the A340 popping in every once in a while. To answer the other question no other carrier flys a A340 to LHR from SFO. No other carrier that operates SFO-LHR even have A340.
BA 2 Daily 747-400
VS 1 Daily 747-400
UA 2 Daily 1 747-400/1 777
"Some pilots avoid storm cells and some play connect the dots!"