Hopefully the OP is already aware that seat refunds can only be claimed after travel, but can be done online so just in case this is the link:

https://www.britishairways.com/trave...seating-refund

I would, too, temper expectations. That's a large reduction in capacity, and although the seat map kindly posted does show a few blocked seats it's inevitable that seating for a lot of people is going to sub-optimal compared to their ideal. I suspect it's quite likely that others will be in a similar or worse position with families split up. By all means talk to the cabin crew, but they will be very busy trying to get a full plane away so their efforts prior to take-off may well be concentrated on getting people sat down in their assigned seats and luggage in the overheads to ensure an on-time departure. All is not lost if your situation isn't resolved by take-off and it might be when things calm down the crew can find more time to try and help you, but clearly the further along the timeline you get the less likely other passengers will be to move if they're settled.