Exit row seating rules can be oddly applied at outstations, I have found.

Coming back from YVR, we (two adults, and two children 9 & 11) had row 63 pre-booked. According to ba.com, the B/J seats (forward facing) are exit rows seats, as are 62A/K (rear facing). 63A/K are not exit rows seats though, so we had the kids in 63A/K and us in 63B/J.

The YVR ground staff were having none of it: all of 63, according to the Canadian air transport authority, is exit row it seems. So the kids were notionally booted out of 63 into 64JK.

The silver lining was that the ground staff blocked 63A/K for us. They seemed to understand that BA's rules differed from the local transport authority's rules, and seemed further to take the view that once the doors were closed it was down to BA to make the decisions. The cabin crew were quite happy for us to occupy row 63 per our pre-booking.