PACE 1984 s24
(1) A constable may arrest without a
warrant-
(a) anyone who is about to commit an offence;
(b) anyone who is in the act of committing an offence;
(c) anyone whom they have reasonable grounds for suspecting to be about to
commit an offence;
(d) anyone whom they have reasonable grounds for suspecting to be committing an
offence.
(2) If a constable has reasonable grounds for suspecting that an offence has been committed, they may arrest without a warrant anyone whom they have reasonable grounds to suspect of being guilty of it.
(3) If an offence has been committed, a
constable may arrest without a warrant-
(a) anyone who is guilty of the offence;
(b) anyone whom they have reasonable grounds for suspecting to be guilty of it.
PACE 1984 s24A
(1) A person other than a constable may
arrest without a warrant-
(a) anyone who is in the act of committing an indictable offence;
(b) anyone whom they have reasonable grounds for suspecting to be committing an
indictable offence.
(2) Where an indictable offence has been
committed, a person other than a constable may arrest without a warrant-
(a) anyone who is guilty of the offence;
(b) anyone whom they have reasonable grounds for suspecting to be guilty of it.
(3) But the power of summary arrest
conferred by subsection (1) or (2) is exercisable only if-
(a) the person making the arrest has reasonable grounds for believing that for
any of the reasons mentioned in subsection (4)it is necessary to arrest the
person in question, AND
(b) it appears to the person making the arrest that it is not reasonably
practical for a constable to make it instead.
(4) The reasons are to prevent the
person in question-
(a) causing physical injury to themselves or any other person;
(b) suffering physical injury;
(c) causing loss or damage of property, OR
(d) making off before a constable can assume responsibility for them.
(5) This section does not apply in relation to an offence under Part 3 or 3A of the Public Order Act 1986.
PACE 1984 s32A
A constable may search any person arrested at a place other than a police station on reasonable grounds to believe that the person may present a danger to themselves or others;
Concealed on the arrested person is anything which might be;
Used to assist escape from lawful custody
Evidence relating to an offence.
PACE 1984 s30(7)