You can instruct with a PPL in two ways...
1) You got your instructor's rating in the old days via the BCPL and instructor's rating route, and now have grandfather rights. You can instruct and be paid. There are still quite a lot of instructors like that around.
2) You prove that you have CPL theoretical knowledge, which effectively means you pass the CPL ground exams. You then do the FI course and pass a Class 1 medical. A lot of people think you don't need a class 1 for this route, but I know someone who's been looking into it, and found that you definitely do (anyone know different, please let us know). You can then instruct, but not be paid. Since the only difference is that you haven't done the CPL flying, there's little point. But it CAN be done.