Eva Air is usually the best value direct Premium Economy service to BKK. I actually wasn't aware of the Thai Prem Econ/ Biz wheeze that sexten described, but be aware you'll almost certainly need two tickets and if you have a delay on the first flight you miss losing your ride to Bangkok without any right to compensation. Given the ultra low cost of flights to BKK I'd probably go direct either in Economy (e.g. Thai), Premium Economy (Eva Air) or when sometimes available on a cheap business ticket via the Middle East (e.g. Qatar Airways or Jordanian). These cheap business tickets are more rare coming out of London than many other European cities though.