Are you insured to drive that vehicle?
All cars must be insured when driving on public roads or even when parked on a public road, but it is not only the car that needs to be insured, you need to check that you are covered by that policy to drive it. Just because a car in insured by someone, does not mean anyone can drive it. You either need to have a policy that allows you to drive other cars (although you are often only covered for third party fire and theft) or you need to be a named driver on the policy.
There may be other factors that come in to play when checking if you are insured on a vehicle. It may be that you have a medical condition that invalidates insurance or that you have only passed your driving test for an automatic car but are driving a manual one.
It is your responsibility to ensure that you have valid insurance in place and that the car is road legal. Even if you as a driver are insured correctly, if there are changes on the vehicle that have not been reported to the insurance company such as aftermarket alloys, full resprays or aftermarket exhausts then you may find the whole policy is invalid regardless. Driving without insurance not only means there will be no pay out should your car or another vehicle get damaged but you can also receive a hefty fine plus points on your licence. It will also make it harder and more expensive to take out insurance in the future.