My understanding is - I don’t have source to quote - that this group of people were disruptive during the flight, so airport security/UK police were involved as the flight landed.
There is a lot more to be discovered from interviews & all evidence captured to give a full picture of what actually happened. Bear in mind that armed police routinely patrol UK airports on active duty to protect national security. Once the civilian police could not contain the incident & the threat to the public increased then additional support would have been brought in to contain the situation.
Armed police are not random ‘plods’ patrolling the high street at chucking out time, they are carefully selected, highly trained, supervised & work everyday at a level of threat that few of us know or imagine. It is standard procedure for any police officer involved in some incidents to come off duty pending formal enquiries.
None of what we have seen online so far begins to tell the full story, we shall all have to wait to find out more. In the meantime, feelings run high & the risk of civil unrest intensifies.