You can instruct as the holder of a PPL under JAR-FCL as you always have been able to under national rules. You do need to have demonstrated CPL level knowledge and pass a FI test on completion of a FI Course. If you wish to be remunerated, then you need to hold a commercial licence.
FIs with only a PPL cannot be remunerated, except for Microlight instruction and Helicopter instruction provided they were qualified before 1 Jan 2000. Sounds like a good equal opportunities case for the European Courts!
A PPL holder only requires a Class 2 Medical Certificate, even to instruct.