Choose a topic outside aviation - the folks on the interview are pilots / instrucxtors / examiners and will rip you to pieces if you get it wrong - choose a non-flying hobby, something from a previous career, anything but.
They're looking for someone who can present a topic in a readily digestible format, not necessarily someone who's got presenting aviation topics down pat. I took in some pre-prepared sheets (1 each for the panel and a fuller set for me with notes on) and used them to talk through my topic, which was something obscure and work-related.
Do a little bit of boning up on the Guild, but you don't need to be able to recite names of Masters from December 1903 onwards.
For me, Helicopter Services had the attractions of Mike Green (don't know if he's still there) and Leon Smith - in a purely aviation "attraction" way, of course
and of not being one or two other schools I could have chosen.