Face masks are obligatory on public transport and about to become obligatory in shops and other indoor spaces. However, not everyone is able to wear one. There is no need to have a letter from your GP regarding this - the guidance has made it clear that members of the public can self identify as being unable to wear a face mask. Please don't contact us asking for a letter.
However, if you are able to wear one a mask, even it if it is somewhat uncomfortable, please try to do so!