Just went for it - Christmas flights home...
14 December BKK-LHR (First) 40,000 + £270. (Annoyingly this one cannot connect to MAN/LBA as it arrives too late)
2 January MAN-LHR-BKK (Economy) 20,000 + £135
£227 taxes altogether.
I only had 60,000 miles to play with, now wiped out, so I am reasonably happy with that. A bought economy ticket on those dates comes to way over £850 with the cheaper airlines and way over a grand with Thai, even in economy.
Brewsterz.