From www.gov.uk/government/publications/covid-19-stay-at-home-guidance/stay-at-home-guidance-for-households-with-possible-coronavirus-covid-19-infection :
"if you live with others and you or one of them have symptoms of coronavirus, then all household members must stay at home and not leave the house for 14 days. The 14-day period starts from the day when the first person in the house became ill.
It is likely that people living within a household will infect each other or be infected already. Staying at home for 14 days will greatly reduce the overall amount of infection the household could pass on to others in the community.
For anyone in the household who starts displaying symptoms, they need to stay at home for 7 days from when the symptoms appeared, regardless of what day they are on in the original 14 day isolation period. (See ending isolation section below for more information."
And from the 'ending isolation' section below:
Ending self-isolation and household-isolation
If you have been symptomatic, then you may end your self-isolation after 7 days. The 7-day period starts from the day when you first became ill
If living with others, then all household members who remain well may end household-isolation after 14 days. The 14-day period starts from the day illness began in the first person to become ill. Fourteen days is the incubation period for coronavirus; people who remain well after 14 days are unlikely to be infectious.
After 7 days, if the first person to become ill feels better and no longer has a high temperature, they can return to their normal routine. If any other family members become unwell during the 14-day household-isolation period, they should follow the same advice - that is, after 7 days of their symptoms starting, if they feel better and no longer have a high temperature, they can also return to their normal routine.