The disability orgs are starting to speak up about this now and several signed a letter today to the NPCC.
An NPCC spokeswoman said that officers have clear guidance on the exemptions and clarified that there is no requirement to provide medical proof of an exemption.
morningstaronline.co.uk/article/b/disabled-people-could-bear-the-brunt-of-confusing-face-covering-laws
If shops are making up their own rules that go against government guidance and that discriminate against disabled people then the way to go is letters of complaint and if that fails, the threat of civil action.
The explanatory memorandum that accompanies the face covering legislation says:
7.6 Exemptions from this requirement exist for members of the public with a “reasonable excuse” for not wearing a face covering. A non-exhaustive list of circumstances in which a person has a reasonable excuse is set out in regulation 4, and generally relate to medical and equalities grounds. ... Nobody who has a reasonable excuse as set out in regulation 4 and is therefore not wearing a face covering should be prevented from visiting a shop or supermarket or other setting covered by these Regulations.
And also:
12.2 The Department has considered the fact that some people may be deterred from visiting the relevant settings where these Regulations apply due to them being required to wear a face covering either because they cannot source a suitable face covering or they have protected characteristics (e.g. a disability) which makes it difficult to wear a face covering ... The Department has also included a range of exemptions to ensure that this policy does not unfairly discriminate against those with protected characteristics. Furthermore, the policy will be supported by a communications campaign that will make clear that some people are exempt from these regulations and people should not be challenged by members of the public for not wearing a face covering. (my bold)
www.legislation.gov.uk/uksi/2020/791/memorandum/contents
And, as posters have noted on numerous threads, the government guidance says that carrying an exemption card is entirely voluntary and people should not routinely be required to provide written evidence of their exemption.