is there any reason why it could not have flown to HKG
Squawking 7700 all the way, presumably ?
Apart from the practical considerations of attempting to fly 5000nm+ with the nose gear down, BA are acutely sensitive to potential criticism of their operational decisions following their infamous three-engined LAX-LHR flight in 2005.
Even if the reason for the NLG's failure to retract had been obvious from an absence of the stowed pin (and so far that's only speculation), the only practical course of action would have been to dump fuel and RTB.