I may be a bit late joining in here, but just incase anyone still interested.
So we went to NZ, the. Stopped in oz for a week before coming back home. 7 1/2 hours to Singapore, then next flight 14 hours from Singapore to heathrow. Just make sure you are well rested before you get on the plane! Emma slept about 7 hours of the flight, but we got hardly any as with turbulence we were having to constantly get her out and back in to basinette.
Yes take any snacks you want. Just make sure any fresh fruit, sandwiches etc are either eaten before you arrive, or left on board. Any packaged stuff is fine. I took water and milk on board. Heathrow and gat wick both have new machines they can put liquids through. No problems at all.
We haven't had to much of a problem with jet lag. We had about 3 days where we were all waking up about 5ish, but we were all tired from all the travelling. So quickly got back to 7pm-7am sleeping. Slightly better than before we went!