It might well have been designed that way, as your cut and paste from Wikipedia suggests, but use of a third seat certainly wasn't ever considered in RAF use. Indeed, it would have been quite impossible to have abandon the aircraft with a parachute for anyone sitting in the rear area - and spinning characteristics would have been very...interesting with such an aft centre of mass.
In any case, before the avionic upgrade in the early 1990s, most of the area behind the seats was taken up by the ancient UHF radio inherited from Chipmunks.
However, the lashing points were useful when there was any need to carry baggage in the back.