If you are in the category of people who must self-isolate on arrival, then when you arrive in the UK, you must go straight to the place where you will self-isolate. If you are not staying overnight you can travel direct to your place of departure (port or airport) to travel back to the Bailiwick.
You do not have to stay in the UK for as long as 14 days (to complete a 14 day self-isolation period there), but you must follow the UK’s self-isolation rules for the time that you are in the UK.
You should minimise the time that you are in any public or shared location.
You cannot have any visitors at all whilst in self-isolation in the UK, unless that person is providing essential care to you.
If you develop coronavirus symptoms when you’re travelling to the UK, you should tell one of the crew on your plane, boat, train or bus. They’ll let staff in the airport, port or station know, so they can tell you what you should do next when you arrive.
More information on the requirements for self-isolation in the UK is available at https://www.gov.uk/government/publications/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk/coronavirus-covid-19-how-to-self-isolate-when-you-travel-to-the-uk.