As long as you'll only be training to PPL level, you should be able to use a Class 2 medical. You are required to hold at least the licence for which instruction is given (FCL.915 (b)(1)). I am guessing that you probably have a CPL, but if your medical is a Class 2 you can only exercise the priviliges of a PPL and instruct for that. The next question is whether you can be paid for said instruction. I'm not sure what the current status is on that topic (see FCL.205.A PPL(A), but I'm not sure if a CPL(A) and Class 2 Medical covers the same situation here. It probably does.). I've based this on EASA Part-FCL and Part-MED. UK regulations should be similar but may change of course.