Ok, I didn't know that. Still, it works another way around than it does in countries like Poland - you have to have your own insurance,to drive somebody else's car. While in Poland you only buy insurance for the car,and anyone with a valid drivers licence is automatically covered by the insurance purchased for that car(unless they are doing it without the permission of the owner, in which case the insurance is invalid).
That's not entirely true, it depends on your insurance. It's an offence to drive a car without insurance, and an offence to allow someone who is uninsured to drive your car. Some insurance policies do not extend the insurance you took out to driving other peoples cars. However, some fully comprehensive policies do have third party insurance while you are driving someone else's car with their permission. In that case it's fully legal to drive.