Section Five of the Terrorism Act 2000 states (edited, my highlights):
41 Arrest without warrant
(1) A constable may arrest without a warrant a person whom he reasonably suspects to be a terrorist.
43 Search of persons
(1) A constable may stop and search a person whom he reasonably suspects to be a terrorist to discover whether he has in his possession anything which may constitute evidence that he is a terrorist.
(2) An authorisation under this subsection authorises any constable in uniform to stop a pedestrian in an area or at a place specified in the authorisation and to search—
(a) the pedestrian;
(b) anything carried by him.
(3) An authorisation under subsection (1) or (2) may be given only if the person giving it considers it expedient for the prevention of acts of terrorism.
45 Exercise of power
(1) The power conferred by an authorisation under section 44(1) or (2)—
(a) may be exercised only for the purpose of searching for articles of a kind which could be used in connection with terrorism
(1) A person commits an offence if he—
(b) fails to stop when required to do so by a constable in the exercise of the power conferred by an authorisation under section 44(2);
For me it’s not so much as to whether they can or will arrest people who don’t comply. It’s more whether any such arrest could possibly be defended in court as being one made the “reasonable suspicion” limitation.
What this law needs is enough brave people to challenge that concept. If I was stopped and asked by a policeman to open my bag for examination I may very well bottle out and do it but I very much hope that there are some brave people who will stand up to this.