This is incorrect. The limitation to essential travel was for third countries outside the EU/EEA and assimilated, with the UK belonging to the latter until the end of the transitional period and therefore not subject to the essential travel limitation. From January 1st, the essential travel limitation would have kicked in as a consequence of the end of the transitional period during which the UK is assimilated to an EU Member-State for a number of purposes, including movements into and within the EU.
The kicking-in of the essential travel restriction now rather than in January is a consequence of the measures taken to manage dissemination of new virus strand.