There is a requirement for a minimum number of hours before you can commence an instructors course (currently 200 hours total with 150 hrs PIC if on a PPL or 200 hours total with 100 PIC if on an ATPL/CPL). Check out LASORS on the CAA web site for full details of the requirements.
It is possible to obtain an instructor rating on a PPL but you are not allowed to be paid for flight training. For this you must hold at least a CPL.
I am guessing here but I would imagine that you would fall foul of the regulations if trying to instruct for free on a PPL. If I remember correctly, you're not legally allowed to do more than share the costs with another pilot so you would still end up having to pay for the flying.
Also, it may not go down too well with the 'salaried' instructors at a club or similar if you were to start offering 'free' instruction.
Basically (IMHO) a CPL is a minimum requirement if you want to build hours by instructing.