There is a VS/SQ RTW available. The last time I flew SIN-NRT-LAX-LHR-SIN, with all sectors on SQ except LAX-LHR on VS, it cost me 2237SGD which is approximately 746 pounds with taxes. However this ticket was issued in Singapore. I would expect it to be more expensive leaving LHR. You might want to check that out. You could so something like LHR-SIN-ICN-SFO-LHR since SIN-ICN-SFO has got the new SQ Y class available.