http://www.legislation.gov.uk/id/ukpga/2004/37

Part 2Enforcement

7Arrest

A constable without a warrant may arrest a person whom he reasonably suspects—

(a)

to have committed an offence under section 1 or 5(1)(a), (b) or (2),

(b)

to be committing an offence under any of those provisions, or

(c)

to be about to commit an offence under any of those provisions.