You can instruct on a PPL in the UK but you must have the required hours as stated above and have sat and passed either the CPL or ATPL exams. You obviously need the FI rating aswell. You do not need to have a CPL however if you dont have one you cant be paid.
Spare a thought though for paid instructors who wouldnt be best pleased if someone came along and offered their proffesion for free.
Foz