In the UK anyone can instruct (I can show my daughter how to fly the aeroplane) but in order for the instruction to count it has to be given by the holder of an FI rating. That rating can be attached to any license so long as the license is equivalent to or higher than the one being instructed for (JAR).
A professional license is only required if its paid for. Absolutely nothing whatsoever wrong with instruction being given by a PPL/FI.
If you are instructing for PPL(M) or PPL(SLMG) you can be paid, even if you only hold a PPL. (have a look at the PPL privileges in the ANO)
There are of course additional requirements that come into play regarding the aircraft (suitability as a training aircraft, registration, certification etc)