Standby upgrades are by sector, so you'd have to do each in turn, and SIN-BKK not worth it IMHO.

BKK-LHR can only be managed in BKK AFAIK. Try checking with staff on Check-in, or call THAI in BKK from SIN to see if standby upgrade is available, and if so, take an earlier flight to BKK.

I think BKK-LHR and vv costs the same, around USD 850.

I started a thread on the costs of these, so if bothered search this forum for thread started by me, and you should find it.....