TG have been 'looking' at MAN for years, and years, and years.
The fact is, a low cost is best suited to this route. MAN-BKK certainly has volume, and I don't think anyone could dispute that, however, the low yield on the route anyway, with 4, soon 5 middle eastern/edge of Europe hubs offering cheap fares between MAN and BKK, it will take a brave and determined airline to offer the non stop flight.
Thomas Cook or Norwegian were originally the best candidates for that, but now NokScoot are in the foray, could be the best option.
The only way we could ever see a 'legacy' airline offering BKK is in the form of a stop over to somewhere else, such as Eva Air MAN-BKK-TPE (not saying they are rumoured, just an example).