So far no-one has asked where to buy bottled water or tinned food. (If I was asked I’d say ALDI). Several people have however asked if I know where they can get a decent shotgun and also whether it should be sawn-off or standard.
To (legally) buy a shotgun in Britain you need to hold a Shotgun Certificate. The appropriate application form can be obtained from your local police station. If you are aged less than 17 you can have a Shotgun Certificate as long as you have your parent's signed permission. The Police may want to see a 'good reason' for you wanting a gun and usually want to see that you will keep your gun (or guns) in a secure place. I'm not sure if defending yourself against looters would count? You have to store ammunition separately.
A shotgun by the way is described as a smooth-bore gun which has a barrel not less than 24 inches with a bore diameter not exceeding 2 inches. The “not less than” bit in the description is of course a reference to “sawn off” shotguns which are easier to carry concealed under your coat and produce a much wider shot spread. This makes them better weapons at close quarters but totally useless for game shooting!