There is no need for a doctor’s letter for this.
A self-declaration exemption can be printed off by patients to carry around via the Travel West Midlands website:
Scroll down to ‘I have a health condition or disability that means I can’t wear a face-covering, what do I do?’ section and follow the instructions.
If you still feel you would like an exemption letter, this could be done by a GP but it does falls under the non-NHS work category and will cost £50 per letter.