Mrs Badger, I don't know if you have previous experience of internet shopping?
If so, I suggest you ring your usual supermarket's help line and ask them about how to get registered with them for priority deliveries.
My parents are with Sainsburys, and at some point must have provided their dates of birth, proving they are over 70.
They were automatically identified by Sainsburys and offered the OAP delivery slots.
Regarding access to the vulnerable shopping hour, most supermarkets seem to be each organising themselves slightly differently.
Some are running healthcare worker hours. Some are running Over 70s hours.
I imagine that this does not help people who are vulnerable but not working in healthcare, or not over 70.
In which case a phonecall to the supermarket's helpline will establish what is going on.
In my house (vulnerable, but not extremely, under 70 years, key worker), shopping with Tescos, we are able to book click and collect or home delivery, but only around 3 weeks in advance at the moment. Or we walk to the nearest smaller branch. I expect more delivery slots will open up as they get more drivers and more pickers in store.
In the meantime, I wouldn't dream of trying to trespass on a 70+ hour, or a healthcare worker hour.