I've never been asked in all the time I've flown with BA.
Nearest I've been was when flying to the US in a BA Y bulkhead seat and there was an Indian couple and an infant already in the A/B seats. They had a family member in the cabin behind who they wanted to sit with. The crew, before take-off, without asking me anything, brought them back and a man forward. So they got to sit together and I had an empty middle seat as he took the window,
AA wise, twice in domestic F.
First time was a transcon on a 321S with TV screens etc. and I was in the rearmost starboard window seat. Arrived to see a woman as my seatmate and her husband across the aisle. They asked if I would swap and I did, taking his aisle seat.
Second time on a flight either in or out of Panama. Two men wanted to sit together so moved from my favoured 6B to another aisle seat 3 rows forward.
No hardship for both flights to be honest.
Once in CX F as well.
Was flying JFK-YVR-HKG back in November and had 1A. Landed in YVR and most people got off.
Then a 30yo or so Asian woman and her elderly father got on. She was in 2A and he was in a middle seat, so separated by the wall / barrier or whatever it's called.
He seemed quite flustered as didn't want to be separated and from what they were saying to the crew, the daughter wanted to keep an eye on him and help him as he wasn't too good on his feet etc.
The cabin crew member asked me if I would swap so I did. Dark outside and was planning on eating a bit and sleeping anyway.