First of all, do not use seatguru. Aerolopa.com is much more up to date and accurate

Secondly, twisting your itinerary to try to avoid a particular seat type is not only unduly stressful but potentially futile. March is a long way off and equipment changes could easily happen, even right up until boarding if there is some technical or weather delay to the aircraft allocated to your flight. Airlines are more concerned with getting you to your destination in the cabin class you booked by that stage than they are which type of seat you are sitting in.

Yes LHR tends to be consistent, A380 or Qsuite. Currently HKT gets one 789 and 2 ex-virgin 777s per day.

BKK gets a mish mash of aircraft. I looked at flights next month earlier today and there was a mix of A380, Qsuite 777, ex-virgin 777 and 788s.

By all means plan as much as you can though. You may benefit from signing up to a free account at expertflyer.com which allows you to search the current seat maps on 5 specific flights per day. A second email address allows a second free account, or there are paid subscriptions.

Planning is all part of the fun of travel of course. Another is that the world isn't controllable, and stuff happens. So my advice would be to give it your best shot and then relax and enjoy the ride, whatever turns up on the day