I think you'll find that senior policemen in the UK command junor policemen and no-one else, not you, me, or any aircraft Captain.
The Police powers of arrest and detention of suspects, for which they must have good reasons, and in certain cases powers to restrict entry to certain places or areas.
They have no powers to "command" an aircraft Captain to detain people in an aircraft, especially one which is the subject of an unresolved threat of an IED aboard. Nor does ATC or the airport operator.
Neither, emphatically, do they themselves have the powers to detain people against their will aboard an aircraft in a possibly dangerous condition.
So, fellas, if you are in the unfortunate position of a Captain in that situation, don't ask, just evacuate your passengers and crew asap. If there are no steps then pop the slides. You are the aircraft commander and you can forget all the stuff about "as soon as it parks you are no longer responsible". If you are then arrested ask them what crime you are suspected of committing, precisely, and then sue for unlawful detention when they have trouble finding one.
On the tarmac and well away from the possible danger, the Police can arrest and detain anyone they reasonably suspect of committing a crime. Others may voluntarily stick around to help them with their enquiries, if the Police ask nicely.
What could make me wrong are any new laws we haven't been told about by good ole Tone to remove our remaining civil liberties and create a Police State in the name of prevention of terrorism. But I doubt very much that even those would specifically require a Captain to detain people in an aircraft against their will when asked to do so by Police, especially before anyone knows for sure if the bomb threat is real.
Don't worry about the SAS. They are rather better trained than the Police, a bit less bloodthirsty, and they rarely shoot people by mistake. They have no civil powers whatsoever to detain anyone.