The cheapest car insurance is free. It will not cost you any money. It's only possible by breaking the law or not owning a car. If you do own a car in the UK it's illegal to not have a car insurance so your next best shot is to get rid of your car or find the cheapest possible car insurance. Your next best option is to start shopping around, compare all online car insurances, get quotes and go for the cheapest if you don't have other requirements.
Since you're looking for cheap you're probably not going for a comprehensive car insurance, so your third party insurance is likely going to be TPO, third party only and if you want to spend a little bit more a TPFT insurance that will also cover fire and theft.
Cheapest car insurance companies
These days the cheapest car insurance companies will mainly work online. They won't be spending a lot of money on fancy buildings, advertising and marketing and like this they can guarantee you a better price. Exceptions can be large supermarket chains such as Tesco. They already have the necessary infrastructure to make it cheap for them to make quotes competing with the cheapest car insurances on the market.
Cheapest car insurance for first time drivers
If you just bought your first vehicle your car insurance will definitely be pricier than what people are paying who have been driving for several or many years. It's actually wise to start with a smaller car that will incur smaller monthly insurance premiums. Even if you've already been driving a lot, this is not going to help you if the insurance policy was not in your name and you will be considered a new driver. Getting the cheapest car insurance will take a bit more effort. This is especially true if you're only 17 year old. Insurance companies are reluctant to insure young drivers under 25 years old.