From 4am (UK time) on Monday 18 January 2021 all travellers to England will be required to provide proof of a negative COVID test. Travellers unable to present proof of a negative test result may not be able to board their chosen mode of transport and could be fined £500 on arrival into England.
The obligation applies to all traveller, including UK citizens, from all destination countries including those on the UK's travel corridors list. The COVID test must have been taken with the three days before the start of the journey to England.
More details here, including which tests are accepted by the UK authorities.
Different rules apply for travellers to Scotland, Wales and Northern Ireland.