An elite group of police officers have the job of identifying each suspect captured on CCTV.
Known as super-recognisers, these policemen have the ability to match faces on camera to those stored in their photographic memory.
This exceptional ability has helped prosecute hundreds of people who took part in last summer's troubles.
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The plaque at number 16 Cowley Street in Westminster marks the brown brick Georgian townhouse where Sir John lived for 31 years.
A man reportedly armed with several knives has been arrested in Whitehall on suspicion of planning a terror attack, police have said.
Metropolitan Police officer, Tom Harrison, has been crawling the course since the race began on Sunday morning.