Here is the government guidance on face coverings:
Exemption cards
Those who have an age, health or disability reason for not wearing a face covering should not be routinely asked to give any written evidence of this, this includes exemption cards. No person needs to seek advice or request a letter from a medical professional about their reason for not wearing a face covering.
Some people may feel more comfortable showing something that says they do not have to wear a face covering. This could be in the form of an exemption card, badge or even a home-made sign.
This is a personal choice and is not necessary in law.
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
