My wife came back with me from Innsbruck a couple of weeks ago with her knee braced and unable to bend it significantly. The accident only happened the day before the flight so we couldn't arrange much in advance. In the end, the airport and BA were very good about it; we had checked in to row 16 (by chance I hadn't moved us forward to nearer the curtain at T-72) which were ones with movable armrests.

For boarding, they took her in a wheelchair to the plane, and transferred her to a special mini narrow wheelchair to take her down the aisle so she didn't have to swing from seat back to seat back. Reverse procedure in London on arrival. Very slick.

If there are two of you, then in extremis you could do what we did; my wife sat in the aisle seat and I was in the middle seat and she rested her leg on my thigh which wasn't particularly comfortable but was just about bearable for the short flight. The third person we were travelling with was in the window seat and would probably have considered herself inconvenienced by my wife's foot if she hadn't been a relative.

I suspect that unless the flight is full you can ask to block the third seat in the row, particularly if you sit near the back.