100% with Heathrow it doesn't need a valid reason. You do as your told.
If I get involved 95% chance you get off loaded if you give any backchat at all.
It is a legal instruction by the crew, and that crew has been delegated by myself within the instructions of their SOP manuals or they may well have asked and I have authorised it to be within the M&B. If you screw with them I will not tolerate it.
If the CC have screwed up I will tell you now that the poo will start descending from a very high height which may end up they are unemployed afterwards. The place is not to argue about it in the air we have zero tolerance for that. On the ground I would be open to complaints if you asked to speak to me. And I wouldn't be that the CC are always correct I would be open to the fact that you may be in the right.
Some airlines the captain wouldn't have the same clout hire em and fire them. But in all airlines if a valid point is raised it would be reported back through the system. The system is god even if your event didn't result in anything happening maybe they just had a bad day but were normally 100% it would stay on record and if there was a history action taken.
To be honest checkin staff are pretty clueless I have had a couple of them had trips to the office as well due to seating allocation putting geriatric gimps in the emergency exit rows. And then it was me that had to chuck the auld
out with them bitching they had paid for the extra leg room and the checkin staff had said it was OK for them to sit there.