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 for 14 days on arrival. This is a legal requirement and failure to comply is a criminal offence. More information is available at https://covid19.gov.gg/guidance/travel. Further information on self-isolation can be found at https://covid19.gov.gg/support/protect.
The self-isolation period in the Bailiwick will start as soon as you arrive in the Bailiwick and will end 14 full days after. Any time in transit in the UK does not count towards that 14 day period. So, even if you self-isolated in the UK for a number of days, you still have to do a full 14 day period when you’re back in the Bailiwick.