Assuming I’m not grief stricken / freaked out in this scenario:
First up - assuming electricity and internet would hang on in there for a little while - I’d research stuff I’d be unlikely to find in books. Find someone who writes a blog about their enviable sustainable, off-grid small-holding type lifestyle. Find out where it they live and get the address so I can plan to take it over. Would need to make sure it had its own water supply. Also research anything dodgy I’m unlikely to read about in books (how to get hold of guns? Drugs?).
Then I’d head into London and live in Claridges for a couple of days. Raid the fridges ad eat fresh lobster, caviar, patisserie etc while I still could because it’ll be a diet of tinned food and whatever I can grow from here on in. While I am there I’d perform some raids on fancy shops - kit myself out with everything from high-end hiking gear and warm clothes through to ball dresses and tiaras (why not).
Then find trucks. Like PPs idea of a post Office Depot (surely we’ll be able to break in and find keys, no need to hot wire anything?) Get to Harrods and load up all their fancy but non-perishable goods while I am still in London. Then a big supermarket and empty shelves of tinned food, pasta and and bottled water.
Locate a warehouse close to my small holding that I can use to store food. I’m going to be aiming to fill it with tinned food over the next few weeks - do some maths on how many hundreds of packs of pasta, tuna, tinned tomatoes etc I am going to get through in 40 or so years I might be lucky to live for.
Then a hospital to get drugs and medical supplies.
Garden centre for seeds and tools.
Find some dogs and cats to keep me company. And chickens. Some goats or cows for milk.
I think that’s a good start?