Average tuition prices for 2016 can be found here www.thetutorpages.com/private-tuition-fees.
For GCSE or Alevel, £30-£40 per hour is not unreasonable for an experienced tutor. Ask the tutor about their qualifications and the results their students obtain. I've tutored for many years now, have a lot of experience, also examine within my subject area, have a history of good results and not only hold a PhD, but I'm on the editorial boards of several international journals in my subject area and have a list of publications as long as your arm. Approximately 95% of my students come from personal recommendation.
Also remember, that from an hour's tuition paid for, travel may be another 30 minutes each way, plus preparation time of another 30 minutes or so. That £30/hr would therefore only be £12/hr of time for the actual tutor, who will then have to deduct costs (petrol, paper, printing etc, etc and because tutors are self-employed, losses due to sickness, holidays etc (whether sickness/holidays of the tutor or the student) will all be at the tutor's cost) and I often do a lot of work for my students outside of the time actually paid for. It always amazes me though that people are prepared to pay maybe £50/hr in labour costs at their local garage, but value education so little that tutors should not expect to receive more than national minimum wage!
Use a tutor with a good reputation, experienced tutors who are good at their jobs won't need to find work via agencies, so if you do use an agency, expect to get a less experienced tutor and/or a student. Also consider a tutor that is full-time, not a tutor doing a little bit of work on the side. They'll take the work more seriously and will be generally more invested in obtaining better results.