Thanks for the update
Any chance you kept your boarding card(s) for the flight(s)? Or do you know if your flight the next morning had a different flight number to the one the evening before?
It could be this nuance that determines if your original flight was operated the following day from a different airport, or not.
I can pretty well guarantee that the Regulation doesn't come close to this particular situation. I can tell you that had you not been diverted to Luton, but had operated ex EMA the following morning that would have been classed as a 'delay'.
The question of when does a delay become a cancellation, if ever, is currently before the ECJ - some NEBs think that a delay MUST be considered a cancellation after a certain period of time. On the other hand the Regulation has a number of references that indicate that delays of more than 24 hours were contemplated when the regulation was drafted. If you ask a charter operator they will tell you that then never cancel a flight - they are contractually obligated to operate even with significant delays. (their obligations are to the wholesalers, not the passengers).
They have some very good people at the AUC, if anyone can help they can. I personally have my doubts that you will succeed in getting compensation - and I do note that it could have been a lot worse. They could have simply cancelled the flight (for technical reasons, no compensation) and then offered to put you on the next AVAILABLE flight whenever that might be. Instead they appear to have put on an extra section (from LTN), put you up at a hotel and fed you.
Good luck!