EC261/2004 does not apply for CX, a non-EU airline, on a flight starting outside the EU (HKG).
If you accept the flight on LH, an EU airline, then EC261/2004 would kick in based on the LH flight's schedule.
If the ticket ends in LHR it would seem reasonable for CX to reroute you on one of its own flights direct to LHR, but you'll need to ask CX to do that.