Yes, you could instruct with a JAR-FCL PPL(A) - but not for money.
You will need:-
1. To hold a JAR-FCL PPL(A) with SEP Class Rating;
2. To have passed all the JAR-FCL CPL(A) examinations;
3. To have at least 200 hrs TT on aeroplanes, of which at least 150 hours are as P1C (if you only have a JAR-FCL PPL(A));
4. To have completed at least 30 hours on SEP aeroplanes of which at least 5 hours shall be in the 6 months preceding the FI course pre-entry flight test;
5. To have received at least 10 hours of Instructional IF (not more than 5 in a FNPT or approved flight simulator);
6. At least 20 hours of cross-country flight as P1C of aeroplanes,, including a cross-country flight of at least 300 nm including 2 intermedate landings at aerodromes different to that of departure
7. To pass a specific pre-entry flight test within the 6months preceding the start of the course.
The FI course itself will equip you to give unremunerated day VFR training only at PPL(A) level with supervisory restrictions. You will need to fly at least 30 hours of instruction, complete 125 hours of ground instruction with 7 progress tests and then pass the FI Skill Test. Later you will need to upgrade from FI(R) to FI - this will mean another test. You will also need other courses and tests if you wish to remove the 'no applied instrument' and 'no aerobatic instruction' restrictions from your FI rating. All this will cost a fair bit - so how would you ever recover your costs without being paid for giving instruction?