Central Flight Training website
Flight Instructor (Restricted) Certificate
The FI(R) course requires the applicant to have either a UK-FCL CPL, or a UK-FCL PPL with more than 200 hours total time, including 150 hours in command and passes in at least the UK CPL theory examinations. In addition to this, the pilot must have at least 30 hours on the class of aircraft on which the course is to be taken, and of these, five must be within the preceding six months.
The course consists of 30 hours of flight instruction with an approved FIC Instructor (up to 5 of these may be flown as mutual time with another Flight Instructor student), and at least 125 hours of groundschool. Upon completion of the course, an assessment of competence must be taken with an approved Flight Instructor Examiner, usually from our base airfield.
The supervisory restriction can be lifted once the instructor has completed at least 100 hours of flight instruction, and has authorised 25 student solo flights.
So no need to obtain a CPL but you do need to pass the CPL exams or higher as you choose in order to instruct for the PPL.