You don't need a CPL to become an Instructor.
However if you only have a PPL and an Instructore rating you will not be able to get paid for Instructing and you will not be able to get your Instructor restriction lifted, which means you will not be able to sign students off solo's or navex's
But you would get free P1 hours from the club that is willing to let you instuct for them..although it will be going round and round in circuits, stalls, slow flight, steep turns and PFL's..