I imagine the requirement for supervision is because they are not prepared to accept a pilot's uncorroborated word when it comes to a qualifying flight for the purposes of a grant of a licence. So although kevkdg may be a qualified pilot & able to undertake the flight on his own authority, the authorities will expect the flight to be verified in the same way as an unlicenced pilot's 150nm flight.
I've been scratching my head over Article 4 (7) for a while. I can't work out whether para (b) limits the whole idea to SEP aeroplanes (excluding helicopters, TMGs, sailplanes etc) or is it that, if flown in an aeroplane, only SEP is allowed?