I'm suprised no-one has mentioned using EVA air on the LHR-BKK route. I'll travelled with them a few times and found them to be pretty good. They are especially good value if you compare their Premium Economy/Business class fares with their equivelents in other airlines.

Only one gripe I have with EVA is that they replaced Evergreen Deluxe with "Elite" - I don't think it's as good a product - although either one still beat Economy any day

Also, they were for a while using 777's on the LHR-BKK route, but they've now gone back to using 747's, so there will a bit more space for stretching the legs, in that area near the bottom of the stairs ! Something I shall enjoy when I travel out to BKK this February