2 (a) He shall not fly such an aeroplane for the
purpose of public transport or aerial work
save as hereinafter provided:
(aa) the giving of instruction in flying, if
his licence includes a flying
instructor's rating, class rating
instructor rating, flight instructor
rating or an assistant flying
instructor's rating;
That isn't the best example, but I can't find the phrase I'm lookng for. This all came up in a thread in the Private flying section a while ago.
The definition of instruction is having an unqualified pilot using the controls under the supervision of a qualified pilot. If you don't have an FI rating, then you can't teach.