I'm hesitant to cite black and white rules due to differing accounts that scholars are discovering while researching historically informed performance. I suggest to students in ABRSM exams that they consider their context. If the passage was composed in a Baroque or Classical Style, I suggest starting trills and mordents above the note, unless it is approached by stepwise motion from above, in which case one might consider starting ON the note. The same would apply to turns, unless it is between notes (when it starts with the first note before executing the turn) Note that Chopin is traditionally the only 'romantic' composer who was said to like to perform ornaments in his music "on the beat", which affects ones choice of starting note as well in some instances.
Look at the context, especially for Grade 8.