How does car insurance work when driving other peoples cars?

I have a 200bhp car and my car insurance provider (quinn direct) allows me to drive other peoples cars 3rd party as long as the cars not in my name. Does that mean I could drive a 400bhp car then and be insured or is it limited to cars under my insurance group? A: Yes you can drive any car and it don't matter what engine is in the car. 3rd party means if you smash up the other car you won't get it fixed by the insurance company so you will have to put your hand in your pocket.  more