But, on FT "dos" the organiser often gets a private room and gives a finite number to the establishment.
I disagree with your wording. I've planned a number of lunch/dinner Do's as well as been to a number of other lunch/dinner Do's, some had buffets, not one has been the set up you describe. I don't think it is normal to have a private room.
Sorry you had a bad experience at the Do you attended, it hopefully should not dissuade any FT'er from experiencing one for themselves.