At least you didn’t have the unclenched masses casually walking forward and occupying the seats! Happened several times on one flight I was on (admittedly not BA). I was getting somewhat irritated particularly since the crew saw it and did nothing, and only acted when I finally pointed out that there was now a young child sitting in the exit row, and wasn’t that a safety hazard?
Although, come to think about it, it did happen one time on BA. I was flying First from BKK (I think) to London, and some brass-necked woman had plonked herself down in my seat and was tucking into the canapes and champers. At first I thought I had the wrong seat but after carefully checking my boarding pass and asking a crew member to check as well, the wretched prole was dispatched back (a long way back) to where she belonged!!