The most important thing to purchase for your car is insurance; there's no two ways about it. If you don't have have car insurance you are breaking the law, and at worst, face the very real possibility of going to prison, and at best getting points on your license if you get caught. Luckily there is no shortage of companies crying out for business so getting a low insurance premium should be no problem at all.
There are several sites that will do the shopping around for you so that you get the lowest possible insurance quote. Moneysupermarket.com allows customers to compare quotes from over 100 of the leading insurance brokers in the UK including Swiftcover, Admiral and Churchill. They even have a really helpful section that advises users how to lower their risks in the eyes of the insurance brokers and benefit from lower premiums. And it's not just premiums you can compare, you can also compare policy options such as legal cover, courtesy cars and breakdown cover, so that you're guaranteed to get the best deal.
Also worth checking out is confused.com which deals with a range of insurance brokers exclusively so you won't find them anywhere else on the web. They also have really helpful features on everything you need to know about your car and insuring it, from no claims bonuses to car depreciation.