England's national lockdown is now law

The regulations enforcing the current lockdown in England were made on 5 January 2021 and came into effect today, 6 January 2021.

The Health Protection (Coronavirus, Restrictions) (No. 3) and (All Tiers) (England) (Amendment) Regulations 2021 (legislation.gov.uk)

These took effect by amending previous regulations:

The Health Protection (Coronavirus, Restrictions) (All Tiers) (England) Regulations 2020 (legislation.gov.uk)

The provisions will be debated retrospectively in Parliament today, and are expected to be passed.