I bought a pre registered, new car from Jefferies Farm a couple of years back. The best thing with them is the range of cars they have on one site. You can see hundreds of different makes and models instead of having to go around all the different show rooms. If you're buying new, then once you've got a price, give a main dealer the chance to beat it. You'll get much better service, especailly if anything goes wrong later.
I paid cash and sold my car privately, usually a good starting point for negotiating a discount, but they weren't budging on the price. I know everyone else has already said it, but don't take their finance.