I just wanted to add something as I have seen some misleading information many pages ago.
I do not know what happens all around the world, but for many european carriers, and definitely for the CAA, any passenger who requires the use of an extension seatbelt cannot occupy
any emeregncy exit seats.
So keep it in mind if you were thinking of booking one of those "old 737s" overwing emergency exits with the extra legroom, you might be asked to move seats. Not because the crew do not like overweight people, but because it's a set procedure that we have to follow.
In BA, if you're flying on one of our short-haul planes, you'll notice that some seats in Y are actually almost the same as the C seats. Try to aim for one of those (window or aisle) when you check in, however if you tend to go a bit "overboard" then it's better to have a seat at the back, where you can lift up the armrest. Club seats do not have this facility. And the Club middle seats used in Y configuration can be a good 2-3 cms smaller than the window/aisle ones, so be careful. I was unlucky enough to get one of those a few days ago: I couldn't complain as I was on stand-by and couldn't move as the plane was full. I used to fit in them nicely however now I am 6 months pregnant and as you can imagine it was the most uncomfortable trip I've ever had.
Hope this helps.
FBW