It depends on your location, the qualifications and experience of the tutor and whether tuition is face-to-face or online. Large agencies may charge more (though don't expect the tutor will necessarily be receiving more money).
I charged £30 (stopped last year). I would ask whether marking (homework, practice papers) is included or whether that will cost you more. I always included it as normal preparation, but have heard of tutors asking extra, which seems really cheeky to me.
I have to say I really dislike the ethos within this industry of some people working without paying taxes owed. I take cash payments from some students, because some prefer to pay by cash, but I still put that money through the books and declare it. It would not be worth my while to risk a tax evasion charge on my clean DBS check.
I charged £35 an hour (outside London) a little while back. I would probably charge £40 now, including marking and providing past papers. I think music teachers charge at least £17.50 per half hour, so that seems fair.