With bad credit history, you wont get a loan at a sensible interest rate.
You'll have to save a few hundred pounds, befriend someone who can (or teach yourself how to) spot the difference between a worn out car and one with a few years of life left in it, and go shopping through your local paper and/or www.autotrader.co.uk. My last few cars all cost under £500 and all were suitable for regular, reliable use.