The UK has released some details about the impending (beginning June 8) 14-day quarantine that will be required of most arrivals in the UK: https://www.bbc.com/news/uk-52774854. There are exceptions for those coming in from Ireland, the Isle of Man and the Channel Islands as well as for a few categories of essential workers. The plan will be reviewed every three weeks. It sounds as if there may later be a possibility of exceptions for arrivals from other origins, but there's no clue as to how soon that might happen.
Those quarantining are not supposed to go out to buy food, etc., "where they can rely on others". Enforcement of the requirement will be via spot checks and £1,000 fines.