The TrainingCom was issued in response to allegations of the selling of aerobatic experience flights being thinly diguised as 'trial lessons'. Since aerobatic training does not feature in the PPL or any other JAA syllabus, there is no excuse for including them in a 'trial lesson', or any other instructional flight.
However, that was not the OP's question. He is talking about a (presumably) private flight in an aerobatic aircraft with another qualified pilot - no instruction involved. In fact, there is currently nothing to prevent aerobatic instruction being given on such a flight, irrespective of the wholly pointless restriction printed on the FI rating. Since instruction is not being given for a licence or rating, an FI rating is irrelevant. The CAA have never managed to get their heads around the concept that you cannot withold a privilege that never existed in the first place.
Of course, this will all change next April with the introduction of the EASA aerobatic rating and the requirement to hold an appropriate instructor certificate (no longer a rating) to give any flight instruction in an aircraft