I would add that in most instances a PPL FI would not be 'employed' by the club/school at all but be a club member.
But if the FI was receiving renumeration then even if they were'nt explicitly 'employed' by the club\school, there would be a tangible relationship between the two which would result in them effectively being considered an employee (as opposed to a worker) should that ever come to the test?
However presumably that's not the case for Self Employed individuals. But self employed FIs would more likely be career FIs/hour builders with the CPL anyway I'd imagine.