What is the most effective way to slow the spread?

London Mayor Sadiq Khan has been on LBC radio this morning talking about – among other things – the idea of a circuit breaker, which is a national lockdown for a fixed period of time (at least two weeks).
“What [the government’s scientific advisers] Sage say is the most effective way to slow down the spread of the virus is to have, for a couple of weeks, a natural circuit breaker,” he said.
But if you look at Sage’s advice, that’s not what the scientific advisory group said.
A circuit breaker is classified as having a “moderate impact” on Covid transmission, with a warning that it “would only apply for a short period and so have limited effect”.
What Sage said would be the most effective way to slow down the spread of the virus was a complete lockdown, as we had in the spring, which it describes as having a “very high impact” on Covid transmission.
BBC News