I do my repeat prescriptions through Boots, and they tend to let me have one when I'm over the review period, with instructions to go see the doctor before I get another. I recommend this because when you go in they've counted it all out already and you don't have to hang around for fifteen minutes.

What they aren't set up to do is e-mail me when I'm due for another, so I tend to forget, which breaks the system.

Pasta keeps for years, maybe decades. Get your doctor to prescribe two months' worth; iterate it through Boots for a year; keep it in an airtight box and think of it as your apocalypse stash.