I think check-in staff need to take the issue of ABP seating a lot more seriously.
I recently had to pull up a check-in supervisor who was about to check a family which included a very frail, 80 year old woman into an exit row (they were the last people to check-in and the a/c was full). I pointed out the lady was not ABP and so the agent moved some other PAX who were already checked in. I am not sure if this was just very poor judgement or if they just couldn't be bothered to do the boarding card swap airside.
On other occasions I have had to refuse ABP seating to people who do not meet the ailines ABP rules only to be told "Well we were given it on the way out". So in this case the check-in staff in the UK clearly made a mistake (which was not later corrected by CC).
The last thing CC need is to have to start moving PAX about once boarded just because ground staff cannot be bothered doing their jobs properly. It is our responsibility to make sure PAX are correctly seated at the time boarding cards are issued!