Taking hold of the controls would be seen as acting as a pilot?
There is no definition of a pilot in either the ANO or Part FCL. The only definition is for Pilot in Command, or Co-pilot. Simply holding the controls does not make you a pilot, anymore than moving the throttle, or sitting in a pilot seat. As there is only one legal pilot in a SPA, any other person operating the controls, other than a FI or FE is not recognised in law as a pilot. Until a court rules otherwise, it remains in no-mans land!