Francis Hoar said:
“Police have a right to enquire but if someone tells them they have an exemption they don’t have a reason to think they’re lying. We do not live in a police state or at least we thought we didn’t.”
If a police officer inquires as to why you are wearing a mask, politely say that you have a medical exemption from wearing one. Should the officer be stupid enough to ask you what the exemption is, politely ask to see his/her identification and collar number and inform him/her that you will make a complaint unless he/she withdraws and allows you to go about your business unimpeded.