The Test to Release for International Travel scheme is for people who need to self-isolate on arrival in England.
Under the scheme you can choose to pay for a private COVID-19 test. The earliest you can take the test is at least 5 days after you left a destination not on the travel corridor list. If the result is negative, you can stop self-isolating.
The scheme is voluntary and applies to those self-isolating in England only.
If you are travelling from a destination not on the travel corridors list, do not have a job that qualifies you for a travel exemption and do not want to opt in to test to release, you will need to self-isolate for 14 full days after you were last in a destination not on the travel corridors list.
The scheme launches on December 15th 2020.
More info here