Earlier when asked about why the govt didn't act earlier the PM said “I think we were told about the new variant and the way it was taking off on 18th December and we went into T4 across the vast bulk of the country pretty much in the next 24 hours.”

This isn't entirely correct.

The new variant was identified in early December after genome data was paired with a curiosity that the rates of infection in Kent were not falling despite national restrictions and then T3.

So there was indicative sense that the variant may be playing its part as early as that and perhaps sport greater transmissibility.

On December 14th Matt Hancock informed the Commons, saying the new variant was in at least 60 local authorities already.

He told MPs in that over that week, there had been sharp, exponential rises in coronavirus infections across London, Kent, parts of Essex and Hertfordshire.

He said they couldn't be sure whether that was due to the new variant but that caution was required.

As a result, London and several other places in the South East were moved to T3.

On 18th December NERVTAG reported that they believed the new variant was indeed more transmissible.

On the 19th, the govt acted. This is the action I assume the PM referred to today.