This is a difficult one. Ask yourself, do I want an experienced tutor or an A Level or University Student supplementing their income? Professional experienced tutors will cost you more but they are more likely to know the syllabus your child will work to and will have a good range of material and test papers for your child. MOST IMPORTANT - have they been checked by the Disclosure and Barring Service to ensure they are suitable to work with young people? This system (previously called CRB checked) ensures that a prospective tutor has been vetted for suitability to work with young people.
In practice, expect to pay around £20-£25 for primary and £25-30 for secondary (GCSE) per hour. A level costs around £35 per hour.
You will find plenty of people advertising that will charge £12-15 per hour. BUT do they pass the tests above. Food for thought!