Privately, in U.K. at least, there is no reason why anyone should not be allowed to teach. With or without qualifications. As Rockin intimates, start with friends, see how it goes. You'll either do it at home, in which case, a room ought to be set aside for it,and perhaps a waiting room for the next pupil or parents, or go visiting, where you'll meet all kinds of obstacles, like teaching in the kitchen, or with others watching t.v. That side of things didn't work for me. Also, the travelling ate into time drastically, and my resources weren't at hand.
As far as your own qualifications are concerned, grading doesn't tell a great deal. You have the experience to put others through a system that you know, if teaching for exams is on the cards. Your theory knowledge should be at least up to grade V, as required by your grade VIII pass. That is useful for when you get asked searching questions. Sometimes the answers may be evasive, but always find the correct ones for the next lesson! I take players through to grade VIII and beyond on guitar and bass, but have not done the exams myself. It's not a pre-requisite, but at the time, these exams didn't exist! (I have grade VIII pno along with other stuff though). Having said that, the question of my qualifications never gets asked.
You may want/need to be on a child protection register,have public liability insurance, etc., and this can be done by affiliating with a body such as RGT, which also has a ton of support for teachers - very useful when starting out.
Some folks are natural teachers, and find it easy. If you struggle, both in the lessons and motivating students, it's maybe a sign...
Another plus is that you'll probably get as much out of it as your students. It's said that teaching is also learning: everything made more sense to me when I taught it.
Given the parameters above, try it out, make lessons weekly rather than more spread out, be adaptable - everyone learns in different ways - keep a sense of humour, foil pushy parents, be prepared to be astounded!