I'm struggling a little with the concept of priority security at TXL - I've never been offered it or seen it in the main terminal. In that sense, even the option to purchase it at Terminal C (which I've never flown out of, so I can't get a sense of the magnitude of the OP's other difficulties there) might be considered an improvement.

I suspect the reason BA didn't offer it is that for 99% of its operations it's simply not something the airport can accommodate. For a single flight which sounds like it had to move to C for unusual operational reasons, no-one is going to put a special arrangement in.