The UK government has issued an official face mask ‘exemption card‘ for people living with disabilities or long term conditions. These are downloadable from the link below, this includes a version (badge) specifically for mobile phones
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.
If you would like to use an exemption card you can follow the link on this page. Those who have an age, health or disability reason to not wear a face covering should not be routinely asked to provide any written evidence of this. Written evidence includes exemption cards – say the UK Government.
Please follow this link to download the exemption cards and/or mobile phone badge.
UPDATE 6/8/20: The Government have now added to its badges and cards that read,
“Please give me space”
which can be used to show the carrier may have difficulties or concerns in maintaining social distancing. Click here for details