That sounds really frustrating for you @Willow, it’s clear that the situation regarding supermarkets has left many people understandably confused!
Have you contacted your local supermarket since speaking to your GP, to inform them of your letter?
There’s a new post on this issue, here, if you wish to take a look - Advice on Supermarket Deliveries
Also, did you take a look at the Covid Mutual Aid webpage that @SuBloodcancerUK posted above? If so, was it useful for you?
Take care, Alice