The Government website has the following section:
https://www.gov.uk/guidance/meeting-people-from-outside-your-household
"If you are clinically extremely vulnerable (shielding)
Unfortunately, we cannot advise anyone who is shielding to form a support bubble at this stage. However, we know how hard it is for people who are shielding and have been keeping this advice under close review. We will be setting out further advice for this specific group next week, now that the peak of the pandemic is past.
Those who are shielding are still advised to maintain strict social distancing and stay 2 metres apart from others – including those they live with".