This is a typically pointless piece of bureacracy, which has no standing in JAR-FCL. It is only necessary to hold a FI rating if the instruction being given is for the purpose of becoming qualified for the grant of a pilot's licence or the inclusion or variation of any rating or qualification in that licence (ANO Article 36). Since there is no aerobatic rating or qualification that can be included in a UK or JAA licence, it is not necessary to hold a FI rating to give aerobatic instruction.
In the case quoted by Pirate, one must assume that the training given to the prospective military pilots was not for the purpose of becoming qualified for a licence (he says that it was for the purpose of 'screening') and, therefore, the instructors concerned would not have needed to exercise the privileges of their FI ratings and any aerobatic restrictions therein would be irrelevant.
A trial lesson, on the other hand, is a part of the PPL course and, therefore, the instructrion is given for the purpose of becoming qualified for the grant of a pilot's licence. The trial lesson must, therefore, be conducted by the holder of a FI rating who must act only within the privileges of that rating.