I stand corrected. A quick look at the dear old ANO shows that aerial work is only applicable to remunerated flights (Art.157) and an instructor's rating is only required for instructing for licences and ratings (art.36).
But the restriction on my FI(A) says "no aerobatics". Does this mean that, if I hadn't got an FI, that I could teach aerobatics? Or does it mean that I can teach them if not logging the hours P1; effectively as a passenger?