But this thread is not about a question of opinion. It is a question of fact. Is someone likely to be liable to a fine or prosecution for use of the emergency egress handle in the situation outlined? Yes, they are as it is misuse of the on-train equipment, as outlined by the railway byelaws. By purchasing a ticket you enter into a contract with the railway which requires you to abide by the laws and byelaws governing the railway. That is the answer to the OP's question.
As others have eloquently stated, all the rest of this discussion about police and detention is just guff. The railway is not breaking any laws by failing to release the doors the instant the train wheels stop turning in order to assist the police in dealing with an on-train incident. All the arguments put up in support are, to be frank, rubbish.
I'm not going to get all holier-than-thou or cry anti-staff bias, but this forum really does make me despair at times. Whenever these topics come around, as they do on a regular basis, I put my head in my hands and question why I bother coming here at all. All these "Yeah, but what if...?" contributions and cod-legal fantasies are unhelpful and only take the discussion away from the real railway into some dream-world railway that does not exist. All it does is to try and winkle out some nugget of faux-logic to justify a certain action or behaviour that in expressly forbidden, and for good reason. Is it any wonder that railstaff like me get so frustrated?
So here are some things I have learnt while working as a driver.
1) It doesn't matter what we do, it's wrong.
2) There is only one rule that applies to passengers: Thou shalt not inconvenience me.
3) We are the willing, lead by the incompetent to achieve the impossible for the ungrateful.
O L Leigh