In Britain at least, you really have to go with a proper driver instructor, because they will teach you exactly how to pass the test, someone else won't teach you the right things required by the test. You can't make any real mistakes in a DVLA test.

But it's still very helpful to practice on a car outside of lessons (paying for lots of lessons can get expensive), and with teenagers that usually ends up being their parent's car (as long as it is something small and not very powerful, unlike America British learners just can't get insured on big cars). But the things parents just won't treat their kids as impartially as someone else would, like an uncle for example, I don't care how easygoing the parents are, there will always be that little bit of tension (with most parents anyway). My dad can be a bit of an ogre when he's in a bad mood.