My instructor said I wasn't ready, I insisted that I wanted to sit the test even if I failed (and if I did I would have said to him that it was time to find another instructor because I'd had a fair few lessons). There was something lacking in his tuition, it was my husband in the end who taught me to reverse park - amazing, kind, patient, and good at explaining things man that he is!
I did a 2 hour lesson with a different instructor about a week before I took my test and he pulled me up on a few minor things like hugging a roundabout a bit more etc but basically said I should be fine if I practice my parking things a bit (which I did and that's where my DH came in)
So I took the test and passed first time. My instructor didn't even say congratulations, he just sort of shrugged and said "well that was very lucky" etc 
I was 25 so a bit older than the average 17 or whatever when I did it. I actually sped a little during the test (not hugely but I went a few miles over for a short time) and dropped a few points but passed. I don't know to what extent the examinor exercised some discretion and thought basically I was ready to just go for it alone.
I'm not sure if he was taking the piss or just a bit crap really (my instructor) but sometimes I think he was trying to string me out a bit. Regular, reliable, never cancelled, could afford it, 2 incomes no kids, I figure he wanted to keep that going but I might be wrong. There was a time where I was driving around thinking I just wasn't learning anything! Also he was a bit creepy, and was always commenting how long my legs were
and then would say "oh I'm not allowed to say things like that"...well don't then! 