Any person arriving in the Bailiwick from anywhere in the world (including, for the avoidance of doubt, Jersey and the UK) is required to self-isolate on arrival. This is a legal requirement and failure to comply is a criminal offence.
In order to to leave self-isolation, everyone arriving from a Category 4 country or region must take a day 1 test on arrival and a day 13 test and receive a negative result for both. If you decline to take either test, you will have to isolate for 21 days.
The self-isolation period in the Bailiwick will start as soon as you arrive in the Bailiwick. Any time in transit in the UK does not count towards your self-isolation period. So, even if you self-isolated in the UK for a number of days, you will still have a full isolation period when you’re back in the Bailiwick.