There is no one-size-fits-all answer to this question, as the best UK car insurer for one person may not be the best for another. However, there are a few things you can keep in mind when shopping for a car insurer in the UK.
First, make sure to compare apples to apples when looking at quotes from different insurers. That means looking at the same level of cover, with the same excess and for the same length of time. Otherwise, you won't be able to accurately compare the policies.
Second, don't just focus on the price. It's important to consider things like the insurer's Financial Strength Rating, their claims handling record, and what kind of cover they offer.
Third, remember that the cheapest policy isn't always the best value. Sometimes, it's worth paying a little more to get comprehensive cover, or a policy with extras like breakdown cover included.
Finally, don't be afraid to ask around for recommendations. Talk to your family and friends, or post in a forum like this one, to see who other people have used and been happy with.
With all that in mind, some of the best UK car insurers according to recent surveys include Admiral, Direct Line, LV=, and Churchill.