Even in the UK, if there is less than a certain percentage of passenger seats occupied on the aircraft, in regards to the A319/20, then there is no requirement at all to have anyone seated by the over wing, self help exits. Quite common on quiet sectors to not have anyone seated in them. The difference being that we did not charge extra to sit in those seats, and if someone asked to move, they were more than welcome, however if no one did, and we were quiet, there was no need to move anyone into those rows.