Gosh... have been a bit shocked reading all this. My husband and I gave up after 7+ entrance assessments and hiring agency tutors (who entirely failed us at 40-60 pounds per hour), so we cut out the family holiday and sold the 2nd car and hired a tuor who was recommended to us entirely by word of mouth.
We paid 120 per hour.
He was a former Head of Department at the #1 prep school that everyone wanted. And yes, his references were impeccable. Not 2, but he gave us a list of 20. I phoned 6, all of whom had children at Eton, Harrow and Westminster. Mostly had earned 13+ scholarships.
So for the price of a nice, a very nice car, we ended up with 3 offers of places (out of 6). Bearing in mind the school fees are going to be at least 180,000 for the next 7-8-9 years, it was expensive, but money well spent. I earn it, I pay tax on it, I did what I wanted with it.
You pays your money, ye takes your chances as my mother used to say. Low money in my experience equally low (zero) chances. High money equalled 50:50. If everyone I read on this site had achieved one offer for every school they went for, I'm sure they'd be ecstatic, especially when the true odds of a place seem to be 10:1 or 12:1 at least.
8 an hour? I think it's what you want to achieve that merits the price.