In accordance with the UK Government guidance and the British Medical Association, Practices are not required to provide letters of support for those who fall under the list of exemptions, or to those who do not fall under the list of exemptions.
In England, you must by law wear a face covering on public transport and in shops and supermarkets as of 24 July 2020, in addition when attending the GP Practice or hospital a face mask must be worn.
The Government guidance suggests there is no requirement for evidence for exemption. It should be sufficient for someone to declare that they are eligible for an exemption direct with the person questioning them (eg bus driver).
For more information please visit the gov.uk website: www.gov.uk/government/publications/face-coverings-when-to-wear-one-and-how-to-make-your-own