As you already know, most people in England now have to wear face coverings in shops and on public transport. However, you do not need to wear one if you have a disability that means you find it hard to put them on and take them off, or because it causes you severe distress. This can include people with a learning disability or autism.
If you feel more comfortable having something to show to say that you don't have to to wear a face covering here are some exemption cards you can use.
These cards can be printed - mencap.org.uk/sites/default...
These cards can be used from a mobile phone/tablet - mencap.org.uk/sites/default...
Remember, you don't have to explain yourself. You don't need to use these cards. You don't need to say anything. This is a personal choice, and is not necessary in law.
For more information on when you do not need to wear a face covering in England here - gov.uk/government/publicati...
I hope this helps a little.
Sarah