Some people are less able to wear face coverings, and the reasons for this may not be visible to others. According to the government website, if you have a reason for not wearing a face covering:
This means that you do not need to request a letter from us about your reason for not wearing a face covering.
However, if you feel more comfortable showing something that says you do not have to wear a face covering, you can download an exemption card template from the government website. You can then print this or show it on a mobile device.