Costs usually depend on the level of tuition and who is doing the traveling. Anywhere between £20-35/hr is fairly normal outside London, depending on the level of tuition (primary, KS3, GCSE, A'level, degree level), the qualifications and experience of the tutor (expect to pay more for a qualified teacher/lecturer or graduate with lots of experience and less for a student).
If you are talking A'level, rather than GCSE chemistry for example, £25/hour is a good deal, particularly if the person holds a (good) chemistry degree (and not some pseudo-science degree, such as forensic science, but something actually recognized by the Royal Society of Chemistry) and teaching qualifications or experience.
Large tuition companies will usually charge significantly more than smaller ones, or individuals (but most of the fees will not be passed on to the tutor, who will probably see less than half of what is actually charged). This is understandable of course, as they have much higher overheads, but don't assume the tutor is getting the full fee from large agencies.