Medications
Stockpiling prescription medications is, in many cases, not so much a matter of space, but of whether you can obtain what you need to stockpile. The medication that you require may be controlled, and thus your doctor may not permit you to have more than a small amount on hand at a time; it may already be suffering supply shortages; it may require careful storage; your GP may not be willing to overprescribe. Or you may consider it too much of a risk (to yourself or others) to keep excessive amounts of medication in your home.
There are no easy answers for this particular topic, sadly. (Well, aside from “stop Brexit“, but this site is aimed at preparing ahead of time.) If you require regular medication but it is not controlled and your doctor trusts you to keep more on hand than usual, then you may be able to get a larger prescription than you normally would or, if you can afford it, you may have better luck with asking for a private prescription. Either way, don’t leave it until the last minute to prepare, and make your GP aware of your concerns (you won’t be the first).
When it comes to over-the-counter medicines, consider what you use regularly yourself, and what others in the household take. Painkillers, anti-diarrhoea drugs, antihistamines, decongestants, drugs for indigestion/heartburn… how often do you take them, how much do you take at a time, how uncomfortable would you be without them? Again, you should consider storage and whether there are reasons it would be inadvisable to have too many on hand. Bear in mind that you may not be able to buy large amounts of a given medicine at any one time — for instance, online grocery shops will typically only allow two packets of a product containing ibuprofen or paracetamol in any one order — so you may need to build up your storage over several purchases. But do make sure that you have enough for your needs; many “simple” over-the-counter medicines are not manufactured in this country.
Contraception is another area to be aware of. If you have a long-term contraceptive solution, perhaps worth just making sure it will not expire in the next few months? If you are taking the Pill, be aware that some brands are manufactured in the EU and may be harder to find after Brexit. It might well be worth investing in a couple of packs of condoms and perhaps even a pack of the morning-after pill.
Toiletries
It’s important not to forget other necessary toiletries. Anyone with babies or small children will probably have realised that they might need a stock of nappies (it may not hurt to buy some cloth nappies as well as disposable ones, even if you do not usually use them). Anyone who menstruates should consider buying additional sanitary protection — not least because stress (stress? why would anyone be stressed? oh, wait…) can disrupt menstrual cycles so you might find yourself needing them more, or at a different time, than usual. Some people may need to acquire additional incontinence supplies. And most people will want toothpaste, soap, shampoo, deodorant and so forth.
Toilet paper is problematic simply because it is so bulky. However, any alternatives (newspaper, anyone?) are likely to block up toilets and make matters worse. Toilet paper is at least fairly light so can be stored on top of other stores or on cupboards, for instance, if there is room.