My current airline of choice when traveling BKK to LHR and return is EVA. A late model fleet,fabulous in flight service and really good departure and arrival times in each direction - day flight to London and night flight back.
Currently the difference in fares between Economy and Premium Economy is 13,500 Baht (£214 at normal rates of exchange but currently £270 due to the pounds' weakness). It translates to about a 30% increase over the Economy Class fare.
Needless to say you have to book very early to secure a seat in Premium Economy but at the price it is well worth it.