This page from Independent Age has tips for getting a supermarket delivery slot, or finding other shops that will deliver food and other essentials.  The information is copied directly from here:

Priority food deliveries

Supermarkets are working with the government to prioritise food deliveries for people who are extremely vulnerable. This means people who have a medical condition that puts them at very high risk of severe illness from coronavirus.

In England, extremely vulnerable people will have received an NHS letter and/or SMS message, and they can register for extra support. This could include help with getting food, other essential deliveries and care. You can register online or call 0800 028 8327. If you’re not sure if you or a person you support are ‘extremely vulnerable’, you can still register anyway. It’s worth registering if you need extra help getting shopping delivered.

Extra help from some supermarkets

Sainsbury’s are working with the government in England to prioritise deliveries for those who are extremely vulnerable due to a health condition. If you live in Wales, Scotland or Northern Ireland, you can call Sainsbury’s on 0800 953 4988 to ask for access to priority home delivery slots.

Iceland are temporarily limiting home deliveries to customers who are over the state pension age, self-isolating or vulnerable.

Waitrose are contacting customers who they know are aged over 70. They are working on a new system for offering older and vulnerable customers priority access to delivery slots.

Morrisons are offering boxes of essentials, including bread, milk and toilet paper. Each box has 20 items and are meant to feed two adults for a week.

Other ways to get food delivered

You could try calling any smaller grocery shops in your area and see if they’ll do deliveries.

Some local grocery shops and Marks and Spencers stores are now delivering a selection of their products via food delivery service Deliveroo. You will need to download the Deliveroo app and see what is available in your local area.

You could also try online meal delivery services to see if they are taking on new customers.

Help from volunteers

Many charities and local community groups are working hard to try and support people who are having to self-isolate. Contact your local council to find out about support in your area, or your local Age UK.

Before asking a volunteer to get food or other essentials for you, take a look at our information about staying safe and avoiding scams.

Independent Age offer advice and support to older people.  They also organise phone volunteers who are good company and want to chat. If you’d like advice or a regular call, phone 0800 319 6789