BA’s seat pricing used to be much more transparent - with the introduction of dynamic pricing it is no surprise to hear customers getting annoyed when they are asked to cough up more cash to action simple requests, such as moving a seat assignment.

I don’t know how much you paid to reserve your seat - prices for ET seats on short haul flights used to be in the range of £10 to £18 pp. With dynamic pricing, the charge now starts at a lower level, from £7 pp which may be what you were quoted at the time of booking. However, it appears to be the case that passengers are now charged higher rates when the request is made after the flights are ticketed or if they later change the seat assignment. What is not clear to me is if the new higher rate exceeds the old fixed rate.