As far as the science and engineering go two points derived from comments to my earlier post.
Water supply is a sensitive issue for me as on my trips I have to either carry (tanks) or make (desalination) water. For the UK the biggest issue will not be supply so much as–potentially–potability. Supplies for water treatment come from where they come from. I don’t know what that supply chain looks like. I suspect that water supply (flushing toilets) won’t be a problem but potable water might be, although probably not. This can’t possibly be worse than the Blitz. The real issue is not bacteria (chlorination, boiling) but toxins excreted by bacteria and viruses. Probability low, impact high. Frankly if the UK is dependent on the EU for water treatment chemicals and does not move very quickly on a humanitarian basis they will be reviled across the world.
I don’t think electrical power will be an issue. Even with rolling blackouts (unlikely) refrigeration will be fine. Even transportation should be okay. The biggest issue is natural gas which is 42% of generation. Where does it come from? Subject to EU tariffs and duties or not?
Frankly I think that a hard Brexit will back the EU against the wall with a soft economy in France, a softer one in Germany, and political ahem concerns in Italy. Brussels and EU member country leaders are trying to bully the UK and like most bullies they are motivated by fear and insecurity. But that’s politics and not why we’re here or what the OP asked about.
I’m not in the UK so I’m not going to stock up on food for it. In your place I would look to the issues of greatest inconvenience for shortfalls. Of the various things I can think of toilet roll and facial tissue (I have seasonal allergies) top the list. I have a lot of Jerry jugs for work so I’d probably get 40 or 60 liters of petrol. I have a number of lamps so I’d be sure to have some paraffin. Wine. We usually have two or three weeks at least of food in freezer and tins/home cans, I suspect I wouldn’t buy more.
I would keep more careful track of prices than I usually do anticipating price gouging (like the dehydrated food “kits” upthread) both before and after 31 October.
Be sure to brush up on the recipe for stone soup. grin https://en.wikipedia.org/wiki/Stone_Soup