Decide on your budget then look here and narrow down your choice of car
http://www.honestjohn.co.uk/carbycar/
If you're buying new then
www.drivethedeal.com will give you some target figures relating to the cost.
If you're buying used, then I found a paper copy of
www.parkers.co.uk a worthy investment
If you have no trade-in car (or selling private) then potentially you can buy from anywhere in the UK (if you are prepared to travel).
for example, 4 years ago I wanted a Mondeo (used). I had no trade-in and didn't need finance. The local ford dealer had the car I wanted for £19,999. I tried to haggle but the best they would offer was £500 off the screen price. Using
www.autotrader.co.uk I found the identical car in a Ford dealer in Manchester for £15,600. Phoned them and put a refundable deposit on the car. Booked a £35 train ticket for the next day and purchased the car - quite a saving.
Last month I bought my wife a 62 reg Yaris. Again, tried a local dealer who could only offer a car at £10,500 (plus he'd only offer £1,400 for our old Yaris). Found an identical car at a Toyota main dealer in West London for £8,500. Paid the deposit and jumped on the train to collect the car. Put the old Yaris in autotrader and sold the same weekend for £2,400
Take your time an think about what you want, need and can afford. Then use the internet to the max.
Good luck!