One of the problems of the Dubai/HK flight is that it is just too short to allow a decent night's sleep. We left about half an hour late last night on the Emirates overnight flight which gets in at around 7am local time. However, the flight itself is only around 6 - 6 1/2 hours so by the time you have eaten you meal and finished watching the film there is only really time for around 4 hours sleep. I was like a zombie when I woke up on landing.
With the London - HK flight(c.12 hours)there was plenty of time to catch a few films and (if you could get comfortable) a decent 6 hours sleep - the timings coming from HK also worked well - most flights leaving in a period between 11pm and 1am local time which got you in first thing London time.
Having said all of that, the 12 hour flight time (not to mention the 7/8 hour time difference) was very hard work and when we used to take the kids back for Christmas it used to ake us pretty much the whole of the trip just to get over the jet-lag. Our present 3/4 time difference and shorter journey time is a walk in the park by comparison.