Welcome to Flyertalk and welcome to the BA forum powerski4me. I hope you have a good flight, and please make yourself at home here.

The seats right against the window on the window side has no arm-rest at all, so that plus the middle seat would give the most space, the place for resting one's arm in built into the door itself. Note, however, the risk of aircraft swaps, particularly if the flight is some way off.