BKK transit:
if you have checked luggage, need to enter through immigration (this usually takes a long time) and custom, pick up luggage, recheck baggage, exit from thailand - min. 3-4hrs
if you have no checked luggage - 1hrs, you can stay in transit area
Feeder: always fly day before (esp. with LCC) - It could be good with BA because it's the same alliance and if it is late, chances are they will be resolved. Remember that anything can happen in winter!
Why not stay in BKK? Le Meridien BKK is a very good property. Great pool, golf course, restaurants - and close to airport, grab/taxi: 150-200THB
Edit:
The return trip may not count towards your 2019 balance! - I don't know BA that much.
Last edited by balage28; Sep 21, 2019 at 3:42 pm