The common theme of recent events appears to be CC on a power trip - pax better do as they are told or else. Not suggesting that was the case here (I don't have a clue); however, if a self-upgrader gets unruly after being told to head back, IF there is a choice between (a) an unscheduled landing, offloading said pax and then returning to the departure airport and (b) allowing the pax to stay up front and have him met by police upon arrival (so as to not incentivize others to try this), the choice seems obvious to me. Of course, events could have escalated beyond a point where (b) was an option, but my expectation of CC would be for them to never allow things to escalate to this point in the first place.
Again, not suggesting BA did anything wrong, but puzzled as to why BA don't put out their side of the story.