People who are British citizens are not subject to immigration control. So a Border Agency Official should not be able to detain a British citizen in the UK.
If that person was maybe involved in a crime, then the police should get involved. There are loads wandering around at any British airport. Although I don't believe the guff that because a person has not gone through immigration control, they can be denied a right to a solicitor. They are not stateless when they step off an airplane.
So Dr Matthew Nolan (Richard Armitage) a British citizen gets sent back to China (not a friendly country) quicker than an illegal asylum seeker to Rwanda.
DC Hana Li (Jing Lusi) seems to be oblivious that the injured Dr Nolan was detained by armed police. Pleading for help with mobile phone cameras recording him. She must have fell asleep in that part of police training where a man is presumed to be innocent until found guilty and has a right to a lawyer.
Especially if being sent to a despotic regime for what could be a trumped up murder charge.
So a clunky start to this thriller hot on the heels of Hijack from Apple TV with Idris Elba. That started off better and then went for a headlong dive.
Armitage must have read the script and thought he was filming another Harlan Coben thriller for Netflix. It does seem to play out like one.
It did improve once moved to the plane journey. Suddenly it became clear, don't eat food meant for Dr Nolan.