The current guidance suggests there is no requirement for the public to provide medical evidence for exemption from wearing face masks therefore a self- declaration should be sufficient.
Please visit the GOV.UK link below for more information on the use of face coverings. https://www.gov.uk/government/publications/face-coverings-when-to-wear-one-and-how-to-make-your-own/face-coverings-when-to-wear-one-and-how-to-make-your-own
or this page for an exemption card that you can print off at home
For specific requirements in reference to public transport, shops or other areas where masks are required please contact those specific companies for further information.