T or K class upgrade BKK-LHR or vv is 50,000 miles, or a little over 5 return K/T class trips at 75% miles earn (2x4500=9000). You might consider paying a little more for q class for 100% miles earn, and if you have time try to include an extended destination for not much more money - and have a stop over in BKK for work inbound/outbound.
So, depending on price, book a multi-city with an easy same day turnaround destination ( e.g. Australia), for example with routing LHR-BKK-SYD (6-12 hours at 'destination)-BKK (stopover as long as required for work)-LHR
At that rate of flying, you should hit ROP Gold and then be better placed...