If you have an age, health or disability reason to not wear a face-covering you are not required to provide written evidence to prove this. However, if you would be more comfortable having something to show in the supermarket etc. you can print off an exemption card from the government website here: https://www.gov.uk/government/publications/face-coverings-when-to-wear-one-and-how-to-make-your-own
Again, this is a personal choice and is not necessary by law.