LASORS is your friend (search from the CAA site). Section H applies.
You don't have to hold a CPL, but you do have to have passed the CPL theory papers and done the 200 hrs flying and the instructor rating itself, so it's unlikely to save you much cash going down this route.
As far as I can see, the major advantage of being a PPL instructor is in the reduced medical requirement (class 2 v class 1).
I'm sure there will be an instructor along in a minute to correct anything I've missed...