A very high standard of driving is needed to get a licence. The lower standard comes afterwards.
I take your point, and you're correct that many people forget it all once they have the bit of paper, but I'm not sure a 'very high' standard is required to pass the test.
I did my test without actually doing a roundabout, for example. And in nice sunny weather. I (and most people) showed my ability to emergency stop ONCE - and with a certain amount of warning, because the examiner tells you that he's about to hit the dash with his clipboard, so you're waiting for it. All in all, you show that on your very best behaviour, you can pass a test. You might be dreadful otherwise.
I didn't even have to do the theory test, I passed before they came in. Although from what I've seen of it, you'd have to be a retarded jellyfish to fail it.
And did anyone else think "oh oh, Citroen Saxo, that'll be some souped up chav...." I hate thinking like that, but I'm afraid it was my first idea...