I think, and I could be wrong on this, that the power is under section 89(1) Terrorism Act 2000.
It seems from what I have read that the pax were not arrested, mearly detained to establish what they knew about a suspect artical.
The above section says:
89. - (1) An officer may stop a person for so long as is necessary to question him to ascertain-
(a) his identity and movements;
(b) what he knows about a recent explosion or another recent incident endangering life;
(c) what he knows about a person killed or injured in a recent explosion or incident.
As it says above, 'may stop a person for as long as necessary...' the passengers were not as such arrested, just 'stopped'.
I am assuming that this is the power used, in any case it fits the bill...albeit the officer in charge may have had to be shall we say, creative to some extent.
The fact that some of the officers were armed is irelevent, local forces have an obligation to have available an armed response to airports, it follows that they would have therefore attended.
Feeding pax is the airlines responsibility, if they did a runner then thats the airlines fault not the old bill's.
Restricted access to the loo would be fairly normal, in that when going the person would have to be escorted...so having only a few police there would slow the process down.