Teaching and Examining are two entirely different things. Anyone can teach the subjects however to examine them you need to fulfill certain requirements:
For PPL Examiners see Standards Doc 11
http://www.caa.co.uk/docs/33/SRG_FCL_11.PDF
and For RTF Examiners see Stds Doc 4
http://www.caa.co.uk/docs/33/pld_std4v05.pdf