Not all exit rows are created equal. On LCA, BA often uses a 321 NEO (exit rows 16, 26, and 27). In that case, 26 is "designated" as exit but has no extra legroom and certainly not any major advantage bar the fact there are only two seats so if two of you are travelling you will have no unknown neighbour. 27 has a lot more legroom than 16, but also note that as both exit rows are in the "back" part of the plane, they all have the thin, non reclining seats, so what you gain in legroom you might lose elsewhere (in my view, if you care about leg room, 27 is definitely worth the tradeoff, but 16 would not be).

If it is a 321 CEO (exit rows 10, 23, 24) then note that in row 24, the extra legroom is only true of the window seats (A and F).

I recommend a look at Prospero's excellent work on Aerolopa for detailed seatmaps. If you do a search, you'll also find a few thread discussing the cabin and best/worst seats on the 321 in our forum which may be useful.