Yellowsun, not entirely accurate I'm afraid.
The grounds to arrest need be no more than a suspicion an offence has been, or is about to be committed.
However, in order to arrest the Police have to satisfy themselves that there is a necessity.
That may be to prevent harm/ damage or loss,
prevent further offences, prevent interference with a witness etc, to prevent disappearance, to allow for a statutory power to be enforced ie samples being taken or to allow for prompt and effective investigation.