As far as I'm aware (please correct me if I am wrong, anybody), the 'teach for free' concept is applicable if you have just a PPL and an Instructor's Rating, albeit without holding a Commercial Pilot's Licence which is required if you are to fly for reward.
I also believe that you may charge your 'student' anything up to half of the price of the cost of that flight under CAA regulations - cannot remember the section but that which states that each pilot/passenger on a Private flight (or instructional without a CPL) may pay up to his share of the cost.
Hope this answers your question,
Jack.