Our local post office/store has got a sign up outside stating that everyone entering must wear a mask and that exempt people must show proof of exemption before entering. It then goes on to list proof of exemption as :
A doctor's note on headed paper
Exemption letter from DWP
Hidden disabilities exemption card
If you cannot wear a mask and do not have any of these then you are not allowed to enter. They have, so generously, said that children under the age of 11 do not need to show proof.
I am not exempt, neither is anyone in my family, and we all happily wear masks no problem. It just doesn't seem right or fair that someone should have to show medical or personal documents to someone in a shop!