This is the annual event where a circuit in central London is closed to cars for the day and bikes rule. The route is open from 10am until 3pm – best to register in advance but it is free. See here
https://www.ridelondon.co.uk/our-rides/ridelondon-freecycle
The organisers say there will be a led ride from the Tally Ho Pub, North Finchley leaving at 10am.