Investigating body?
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Will the UK AAIB be involved in this as the flight originated in the UK or will the fact that a stopover in HKG was involved override this?
Yes, but probably not as the Lead.
Most likely Australian or possibly the state where it landed since a lot of investigation needs to be performed in situ.. Who leads it should have no bearing since the UK, Australian and US investigators will probably all particpate with experts and provide recommendations for prevention.