What you need to do when you arrive in the UK depends on which countries you have travelled through in the previous 10 days as per the Government's 'Traffic Light System'.
Arrivals from ALL countries must complete a Passenger Locator Form and provide proof of a negative Covid-19 test result taken within 3 days of your homeward ferry or Eurotunnel crossing. Everyone aged 11 years and above must provide a test and the transport company will check the test results before boarding is allowed. Covid tests must be of a certain standard and it is the traveller's responsibility to ensure they get a test that meets these requirements, see Covid-19 testing guidance for details.
In addition: Arrivals to the UK from 'Green' countries must book and pay for a Covid-19 test to be taken on Day 2 after arrival.
Arrivals to the UK from 'Amber' countries must self-isolate for 10 days after arrival and also book and pay for 2 Covid-19 tests to be taken on Day 2 and Day 8 of self-isolation. On arrival they must travel directly to their place of self-isolation and stay there for 10 days.
Arrivals to the UK from 'Red' countries must pay to quarantine in a managed hotel facility for 10 days after arrival and also book and pay for 2 Covid-19 tests to be taken on Day 2 and Day 8 of mandatory hotel quarantine.
Full details can be found on Gov.uk