I've heard rumours that you will be able to get an FI rating on just a PPL. Not 100% sure if thats going ahead, perhaps it is going ahead or is already being done.
However, even if you can instruct with a PPL and FI rating, without doing a CPL it would be illegal for you to earn money by that method of instructing. So that'd be something to bear in mind.
I think I'm right in saying that, happy to be corrected if thats not the case!