Count me in the camp that says that if you're really starting in Blenheim, you may well be better off flying to WLG than CHC, particularly if you do the very quick flight over to BHE from WLG. However, this will depend in part on where you're thinking of going and the car rental requirements.

If collecting Avios is important for you, then don't forget that your husband (if he's the only BA Silver between you) will collect a great deal more Avios flying on BA flight numbers than on QF flight numbers because of the different status bonus awarded by BA on BA-coded flights and QF-coded flights.

Also, I wouldn't let the following should sway you too much, because the LHR-DXB sector is, if anything, even for sleep than the SIN-SYD sector as LHR-DXB is an even shorter flight. The QF flights from LHR to DXB are currently scheduled to depart at 2130 and 2230, so your body will be wanting sleep during the first sector - so there's a good argument for making the first sector the long one so that you can get some decent sleep when your body is wanting to sleep. If you fly LHR-DXB leaving at that time, you might get 2-3 hours sleep on the first sector and then be woken up when your body thinks it's 3 am - and this is very painful.