You do not understand the new BA seating policy. I don't necessarily blame you because of the way BA has publicised this, but you don't understand it.
YOU AND YOUR PARTNER ARE ALREADY SEATED TOGETHER. You are on the same booking, I assume, so the BA computer will give you 2 adjacent seats automatically in 95% of cases.
The only reason to even contemplate paying the £240 would if you wanted a window pair, but for a short flight like this I wouldn't even bother paying that.