Onan, in the JAA associated countries (UK being one of them) it is (for some strange reason) possible to hold an Flight Instructor rating on a Private Pilot License having passed atleast the CPL written exams. You don't need to hold an actual Commercial Certificate, or an Instrument Rating, which you do for an CFI-airplane in the US system.
GodisMyCopilot, A job as an instructor, having only a PPL and the passed the writtens, is not possible i believe. Simply for the reason that you hold a Private Pilot Certificate, and not a Commercial Pilot Certificate, so you are not able to get any form of payment. (someone correct me if i am wrong)
Second of all, even if you start teaching for free, the question is, will you get a lot of students. Having so little experience, you might run into trouble with students not willing to accept things, or not feeling happy or even safe (justified or not) with you as an instructor.
-IBLB-