Hi there,
Hopefully I can answer some of your questions.... I have been flying TG for quite some time and have experianced F class and what it is like.
1) You are flying on TG..... That means you can choose seats and reserve through TG, not BMI. Give them your reservation code and if unknown quote your name and date of travel.
2) If I was travelling with a partner, I would definitely take either seat pairs 2EF or 3EF. But considering you like window seats maybe you should stick with your deccision of 1A and 1K. The seats are not that far apart from each other and you should be able to maintain a chat thats not to loud and not to quiet.
3) When you arrive at LHR check-in make sure you raise the fact your are transiting through to HKT. Your luggage will be ticketed to go through to HKT. There is no nead to clear customs in BKK, as you are in transit and will clear customs when you arrive in HKT. Also in LHR, ask on what aircraft your flight is for BKK-HKT and request your prefered seats. You should be given your boarding passes for BKK-HKT in London. But in the unlikely case you are not they will be available from the TG transit desks in BKK.
4) For your HKT-BKK flight on the 744, you can request to be seated in the F section for your J class flight. Make sure you arrive early (maybe 3 hours) and are very pollite and pleasent to the check-in agent. Ask they can release "rows 1-3" stating you would like to be seated in the F class section of the aircraft. They should allocate you a seat in the F section. In the unlikely case the flight is not very full, they will not seat passengers in this section as they believe there is no need to. But that is not common as people tend to try and take 744's for domestic flights and the J class is mostly booked up. Do note that you will only really be getting a bigger and better seat. You will still be given the same meal, drinks, hot towel, IFE and service as those who are seated in the regular J seats. But still you have the better seat!
Hope I have been helpful to you and do let us know how you go with reserving you F class seats.
Cheers.