Government advice on the use of face coverings can be found here: www.gov.uk/guidance/coronavirus-covid-19-safer-travel-guidance-for-passengers#face-coverings.
This link includes a list of reasons why someone might be exempt from wearing a face covering.
Please note: GP’s are not in the position to provide risk assessments or letters for patients who are worried about the new government guidance and feel they should be exempt from wearing a face mask.