Unfortunately BA are under no obligation to through check your bags onto Thai – even if it was with another one world carrier they wouldn’t have to do it (or even another BA flight) if issued as separate tickets. It is possibly much cheaper to have issued them separately but you lose out on the “protected” connection and the through checking of baggage ability (unless the airline decide to do it anyway as they do often do).
Likewise the airline can always change seat assignments without notifying you for a whole host of reasons – often because a plane has been changed. I agree it would be better if they at least had the grace to tell you – but they often happen at the very last minute. It is a pain when I have organised clients on 2 separate flight reservations and organised seats together, only for people to be moved. If you complain you may get a response along the lines of the following – which my client got
“Unfortunately, there are occasions when we are required to make seat changes. This can be due to many reasons including a change of aircraft configuration, to accommodate a medical passenger or to sit a family group together. As such, we do not give 100% guarantee that all seating requests can be met and we do state this in our conditions of carriage.”
Probably not what you want to hear – though no doubt what you have epxerienced is not uncommon or frustrating.
Have a good trip back.