The only dedicated site I am aware of for online tutors is Spires. We used them for a level chemistry and all their tutors seems to have been at oxbridge. Quite reasonable as well, I paid £20 per hour for a lovely girl called Ellie and we live in the sticks so impossible to find a tutor of her calibre locally. Their online platform is really swish too. Highly recommend it. I think their URL is www.spires.co
I teach higher education students. The only agency I advertise with is First Tutors that have brought me lots of clients. You don't have to pay to register as a tutor, but the student has to pay for your contact details. I find them very easy to work with, and I have been with them since 2008. You can offer primary school right up to higher education, your details are verified by the agency and you can elect to do online/ off line teaching.
I work for a company called Satchel. We are the team behind the homework software Show My Homework also have a tutoring service, Satchel Tutoring. All our Tutors are fully qualified, DBS checked teachers and we always looking for new Tutors to join our service. Please take a look at our website. You can apply to be a Satchel Tutor online there.