The gov.uk website states: https://bit.ly/3fqHtd4 “If you have an age, health or disability reason for not wearing a face covering:

• You do not routinely need to show any written evidence of this
• You do not need show an exemption card”

GP’s cannot provide exemption cards or letters but they are available to download here: https://bit.ly/3uYiSTf

For more information https://www.gov.uk/government/publications/face-coverings-when-to-wear-one-and-how-to-make-your-own/face-coverings-when-to-wear-one-and-how-to-make-your-own#exemptions