First off, I'm not trying to panic anyone here. Quite the opposite: get a plan B lined up, and it will take away some of those "what if...?" fears that keep you awake at night, because you know you'll be fine in the worst case.
Nor is this a lock-the-doors-and-buy-a-shotgun post. The Zombie Apocalypse probably isn't here. Up to a point, European governments have sufficient resilience and resourcefulness to ride out a major crisis and keep the basic services going, even if they're making it up as they go along.
I'm just offering a few simple ideas based on my experience in the less salubrious parts of the globe, where things are deliberately set up to go real bad, real quick (because fear keeps the plebs in their place). These are things you can amuse yourself with instead of going crazy with boredom while you're stuck in the house, and they'll all be useful even if the worst never happens, especially if you have some outdoorsy hobbies.
1) Arrange potable water. Sawyer, HydroBlu, and a few other companies make "emergency" water filters. These are very effective - you can take almost anything, filter it, and drink it without risk - and they don't cost much.
2) Arrange a water supply. Despite Britain's reputation as a rainy country, annual precipitation is only 800-900mm (400mm is considered 'arid'). You need to collect water from your roof. Maybe you already do, but if you don't, find a big container and set it up to collect roof gutter runoff. If you're handy with DIY, you can set up a "first flush" system on your downspout, but it's not critical. Even if you never have to drink it or bathe in it, you can use roof water for the plants.
3) Grow food. You should still be able to get seeds from the Internet. It's hard to recommend ideal plants for beginners because some areas of the country are unsuitable for certain crops, but I suggest trying salad vegetables, crucifers (cabbage family - including turnips/swedes), "walking" or "bunching" onions, beans (not broad beans), and garlic. All of these are easy to grow and take up little (horizontal) space. If you're growing in pots, leaf veg and carrots are a good bet - you can buy seed for stubby carrots that don't need much depth.
4) Make food. Anyone can make ham, yoghurt, and pickles; they're incredibly easy and a lot of fun. They'll also be of superior quality compared to supermarket rubbish.
5) If you have £1000 to spare, get yourself set up with solar power. This is not difficult if you or someone you know has a bit of competence with electrics. You need solar panels (£500), a controller (£150), batteries (£200), and an inverter (£100). All these bits come with instructions, and there's not much to it except connecting one thing to the other thing. I'm an electronics engineer, so possibly I'm underestimating the challenge here, but a bit of research may reassure you. You can just install the panel(s) on your lawn - no need to put them up on the roof, as long as they're not shaded. As well as running a freezer (see below) you can also set up a separate lighting circuit. I do not recommend installing an automatic transfer switch, or interfering with your house mains wiring in any way, unless you're a qualified electrician. Keep the backup separate.
6) Most people prep with dried and tinned food. This is a mistake - you could end up eating a lot of stodge, and most likely unappetizing stodge. Run your freezer off solar, and stock it up with meat, sausages, butter, UHT milk, and cheese. A freezer and solar panels justify each other. A modern chest freezer uses about 1kWh/day, so size your system for that: 48V (4pcs x 12v 30-40Ah) lead-acid batteries, 3 or 4pcs 150W panels (60V nominal at maximum power point), 500-1000W inverter. If possible, buy a spare 24V controller and inverter just in case some of your panels and/or batteries fail. Use a timer switch to switch off the freezer during the night (it will still hold its temperature if it's full) to preserve battery lifetime.
7) Buy a rocket stove. If things get so bad that the water supply stops, power and gas will be off too. Some people like to build these stoves in the backyard, but in my opinion it's not worth the effort when you can buy a nice one (I have an EcoZoom) for £100 or so. Obviously, make sure you have a good supply of wood, too - they're efficient, but they burn surprisingly fast. If the Zombie Apocalypse doesn't come, they're great fun for camping and daytrips.
8) Perhaps the most important thing of all: get healthy. The COVID-19 epidemic is making it abundantly clear that people who are overweight, (pre)diabetic and have heart disease are at great risk, and these issues are all rooted in poor diet: far too much bread, pasta, rice and sweets, and far too little fresh veg and dietary fat. Now is the time to get this sorted.
