If you have been in close contact with someone who has tested positive for Covid-19 you must self-isolate. 

if you are contacted by the NHS Test and Trace service because you have been in close contact with someone who has tested positive for coronavirus you will be told to begin self-isolation for 14 days from your last contact with the person who has tested positive. It’s really important to do this even if you don’t feel unwell because, if you have been infected, you could become infectious to others at any point up to 14 days. Your household doesn’t need to self-isolate with you, if you do not have symptoms, but they must take extra care to follow the guidance on social distancing and handwashing and avoid contact with you at home. Full details of how test and trace works can be found on the Government's website.

It is now law in England to self isolate if you test positive or are contacted by NHS Test and Trace. More information on this can be found on the Government's website.