Time: unless you’re lucky enough to have a friend who’s a qualified driving instructor, it could take longer to reach the required standard necessary to pass your test.
Driving standards: a qualified instructor will have been regularly assessed to ensure that they’re able to offer a high standard of tuition. Importantly, they’ll also avoid teaching you any bad habits, while you might find that you pick these up from your friends, especially if it’s been some time since they passed their test.
Knowledge: a driving instructor will know all the current road rules and be able to provide you with the latest updates. Because qualified instructors know what you’ll be examined on in the test, they structure your lessons accordingly. Your friends might think they know everything, but their knowledge might not be up to date.
Safety: driving school cars come with dual controls, allowing the instructor to quite literally step in if required. Obviously, your friend’s car won’t be fitted out in this way, so you’ll need to be extra careful.
One thing to think about might be combining approved lessons with some top-up time from a willing friend or relative. This can be an ideal option with fewer paid lessons and plenty of road-time practice, while still getting professional tuition.