I think, and I could be wrong on this, that the power is under section 89(1) Terrorism Act 2000.
and
...albeit the officer in charge may have had to be shall we say, creative to some extent
The officer in charge would have had to be very creative. Section 89(1) applies to Northern Ireland, not the mainland.
It seems from what I have read that the pax were not arrested, mearly detained to establish what they knew about a suspect artical.
Under English law, unless there is a stautory exception, you are either under arrest, or you are free to go about your lawful business. There is no inbetween. Being "merely detained" is something that happens in Zimbabwe. A police officer should know this, it is part of his basic training.
As to what power there was to detain these people, the answer is probably none. But if there were, common sense would require that they should be given an explanation, and provided with food, drink and lavatories by the people detaining them. To suggest otherwise is ridiculous.
In these circumstances, most people would probably be happy to wait and assist the police if the position was explained to them by the police. But ultimately, a passenger in these circumstances would have been entitled to say to a police officer: I now wish to leave; you must let go about my business or arrest me.