FI(r) and FI
You do not need a CPL to be a Flight Instructor. The FI can be attached to your PPL, CPL or ATPL, whatever. However, you cannot be paid unless you hold a commercial licence. You may instruct without payment while holding only a JAA/EASA Class 2 medical.
You may also wish to look at becoming an FI Microlites. You may instruct for payment on Microlites holding only a PPL and class 2 medical. Some of the types are hardly different and the pay is usually better as well!