Getting car finance can be difficult if you have a bad credit rating. At the other end of the spectrum if you are buying a new car and you have a credit rating above a certain level then you can benefit from 0% car finance deals.
You can also take advantage of the UK scrappage deals and get several thousand pounds for your old car, much more than you would otherwise get for it. Being able to get an interest free finance deal means that you can literally save thousands on interest charges.
Many main dealerships are able to offer these deals as they want to sell cars and offering to let you drive away a brand new car and pay then back over 3 or 4 years means that they achieve their targets.
It may be worth approaching a main dealer at the start and see if you do qualify for a zero interest offer. Even if you do not qualify it is often worth checking. If you are not desperate for a car it may make sense to rebuild your credit rating as it will save you thousands in the long term.