From the above link updated today
We are advising those who are at increased risk of severe illness from coronavirus (COVID-19) to be particularly stringent in following social distancing measures.
This group includes those who are:
aged 70 or older (regardless of medical conditions)
under 70 with an underlying health condition listed below (ie anyone instructed to get a flu jab as an adult each year on medical grounds):
chronic (long-term) respiratory diseases, such as asthma, chronic obstructive pulmonary disease (COPD), emphysema or bronchitis
chronic heart disease, such as heart failure
chronic kidney disease
chronic liver disease, such as hepatitis
chronic neurological conditions, such as Parkinson’s disease, motor neurone disease, multiple sclerosis (MS), a learning disability or cerebral palsy
diabetes
problems with your spleen – for example, sickle cell disease or if you have had your spleen removed
a weakened immune system as the result of conditions such as HIV and AIDS, or medicines such as steroid tablets or chemotherapy
being seriously overweight (a body mass index (BMI) of 40 or above)
those who are pregnant