Having car insurance is a legal requirement to drive on the UK public roads. The minimum cover you can buy is Third Party, but some insurers won't sell you this cover on its own as it does not cover the insurer in cases where the car is stolen or where the fault lies with the insurer. It may also be the case with low cost car insurance quote.
If you are involved in an accident and it was proved your car was unroadworthy your car insurance company may decide not to pay out.
A lot of car insurance companies are now looking at customer's credit ratings, so if you have a good credit score you may benefit from lower premiums.