First and foremost, while this issue goes to comfort, it also goes to health and safety. As a health and safety issue, it ought to be addressed by the crew in the same way the crew would address someone smoking with a laptop open on a tray table during takeoff or landing. However, it is not always quite so obvious and it may not be directly related to weight or height, but rather "width". A very tall and thin person will be uncomfortable, but a short and fat person may impinge on the next person's space.
In the first instance, this should be dealt with before boarding if possible. If not, crew need to be aware of it. If not dealt with, it befalls the "victim" passengers to bring it to crew attention.
Crew need to deal with it before the aircraft moves, just as they would deal with a person who can't walk in an exit row. It is neither a criticism nor a discriminatory move against any person and that is why BA, and most carriers, have a specific standard.
If the passenger doesn't fit, it is BA's discretion as to how it makes the situation safe. If there is no safe situation, the passenger must be offloaded and the situation deal with anew.
Turning this into a compensation and "goodwill" issue diminishes the safety aspect. Similarly, one should be equally unmoved by "densifying" arguments and the like. It may be that something ought to be done about that issue, but the solution to that issue is not to violate safety rules in another area any more than people who complain that they ought to be permitted to carry steamer trunks onboard because they are dissatisfied with the speed of luggage delivery.