I would advise this:
1. place a small advert in a local paper or post office/shop now. Use this to gauge interest. You may be shocked to realise that your phone doesn't ring at all.
2. You don't have to register as self-employed with HMRC as soon as you start. So you have some time to play with. Visit the HMRC site for the guidlines.
3. Before you pack in your day job, consider any credit cards, mortgages etc. that you might wish to change as after you leave regular employment, you may find it harder to do it. Or if you need an extra credit card, get it now hilst you can.
4. Save like crazy for 6 months and tuck a bit away with your Mum/ other relative for when things get desperate.
5. Don't open a busines account until you have something to put into it. You'll need the free banking for as long as possible when you start trading.
6. Shop around for free business start up advice. Streets Accountants offer this for example and there may be other organisations in your area who can help you through the various stages. I know one person who reckons the best thing they ever did was join the Federation of Small Businesses fore example and got great advice plus deals on bank account, credit card payments etc.
7. Don't blow your money on van and tools. You can make do with a normal car with roof rack at a push. Simply buy magnetic door signs for about £50/pair.
8. You'll need to join a Part P Scheme. This will set you back about a grand or so to include fees, insurance, Regs and tester. Apply once you start to pick up a few jobs and start to get enquiries. You can always pop around to give estimates for the practise and say you are too busy if you can't do it.
9. Don't print thousands of marketing items such as business cards, letter heads etc. Go for freebies from Vistaprint for starters.
10. keep your day job and try and pick up evening and weekend work now the days are getting longer. If you pick up a big job, simply notify it the LABC first and pay the fee. It's profit lost but it gets you going.
11. Register as a sub-contractor with HMRC under CIS so that you can sub yourself out officially if required.