This all seems legal
Under s9 of the LAW ENFORCEMENT (POWERS AND RESPONSIBILITIES) ACT 2002:
(1) A police officer may enter premises if the police officer believes on reasonable grounds that--
(a) a breach of the peace is being or is likely to be committed and it is necessary to enter the premises immediately to end or prevent the breach of peace, or
(b) a person has suffered significant physical injury or there is imminent danger of significant physical injury to a person and it is necessary to enter the premises immediately to prevent further significant physical injury or significant physical injury to a person, or
I haven't seen the film but courts tend to be generous when interpreting "reasonable grounds" so unless the officer's suspicion is clearly nuts, the entry was legal. The leading case is R v Rondo where the court said:
reasonable suspicion involves less than a reasonable belief but more than a possibility. There must be something which would create in the mind of a reasonable person an apprehension or fear …A reason to suspect that a fact exists is more than a reason to consider or look into the possibility of its existence.
However, under s9(2) they may only stay on the premises "as long as is reasonably necessary in the circumstances."
Further, while Part 9 of the act gives protections to people who are under arrest, these do not apply generally. That is, a question that would be off-limits to someone who has been detained is perfectly legitimate to someone who hasn't been. That person can choose to answer or not (they must give their name, address and contact details if asked) and they can choose to stay and be questioned or not because they are not under arrest.
As for the use of force, s230 is very generous:
It is lawful for a police officer exercising a function under this Act or any other Act or law in relation to an individual or a thing, and anyone helping the police officer, to use such force as is reasonably necessary to exercise the function.
What if it isn't?
The biggest consequence of an illegal search or questioning is that the court may (but is not required to) exclude any evidence obtained at trial. See What do I need to do about police unlawfully break in my house