If you were a police officer what would you rather do: sit in the cold outside the house of a known extremist all day, or sit behind a desk with a cup of tea and scrolling through Twitter?
In May, just after the second of four Islamist terrorist attacks in the UK so far this year, British intelligence officials apparently identified 23,000 known extremists in the country. Of these, around 3,000 are believed to pose a present threat and are under investigation or active monitoring. The other 20,000 are categorised as posing a "residual risk". Due to the strain on resources, those 20,000 are not under constant observation.
This is a subject which, since the terrorist attack in May, has caused some agonising among the British public, not least because of the identities of the attackers.