I've been using lever arch files and plain, cheap dividers for years now. If you start with a couple of lever arch, and loads are on sale right now, then you can do your own categories and expand as and when you need to. Shouldn't cost you more than £6-£8 I reckon. For things like guarantees and the Car etc, I have plain carboard folders and go through it every so often to chuck things that are no longer relevant. Also have a very small folder in a partic draw for "Important things" so driving licence, passports, birth certificates, savings book, red books etc.
Because I've been doing it for so many years and have to keep records as self employed blah, I have separate Lever arches now for:
My Banking (includes savings, current, Credit card etc), DH Banking, Tax, My Work (invoices, receipts, mobile phone bill etc as I'm freelance), Utilities (separated into Gas, Electric, Phone, Water, Council Tax blah) and DS general file. Before I get into filing and sorting, which I probably do every three months, I have very cheap plastic folders with same labels on so I can just stuff things in on a daily basis. Then I hole punch everything and file in date order blah blah. If there's a crisis in the meantime you can find the last three months in a smallish folder and before that it's all date ordered.
Sounds very dull, and I am deeply, deeply anal about all these kinds of things, but if you ever have a dispute with a company, move house, want to change utilities or whatever, you can work out where you are and what you spend/owe, really really quickly. Has saved my arse a million times. We keep all the files in a cupboard, hidden away!
Does this all make me really sad?