It's not my country, so I won't dare suggest what they should do, but just like the investigative work in the aftermath of the Manchester bombing, police have already rounded up 12 terrorists involved in yesterday's attack.
It seems that the police knew what these people were up to. I suppose it would have violated their civil rights had they been watched too closely. That's certainly our own approach in this country, where, for instance, the NYC police have been ordered by Mayor De Blasio to stop monitoring mosques.
UPDATE: Piers Morgan says there are an estimated 23,000 terrorists loose in England (not the 3,000 I said previously), and when she was Home Secretary, Thersesa May cut 20,000 police from the force. Morgan feels that was ill-advised.
He also thinks it's time to arm British police officers, which seems like a sensible idea to me — a bobbie armed with just aa baton was stabbed in the face when he tried to stop one of the bastards, and only when armed policemen arrived was the Ramadan celebrant shot dead.