In years gone by I have seen several tall men upgraded on board after complaining about needing more legroom. It rankled a little, but I wouldn't have dreamt of complaining to BA about it. Maybe I should have asked to be upgraded because I needed more butt room!
As for the wheelchair issue, the easiest and fastest way to provide assistance to passengers who need it is to stick 'em in a wheelchair and push them around. Plenty of people who can move around the plane quite easily are brought to the plane in wheelchairs because they need
some assistance.