We've just done exactly what you are planning; travelled from Australia in March to the UK with our 3 kids (admittedly, a little older than yours - ours are 7,9 and 11) and have been on the road ever since. I've travelled long haul myself several times and most flight companies do a trip to Singapore then a long-haul (17hrs or so) direct to London or similar. Best thing we did was to travel on Qatar Air, several of my friends have done the same with kids and also had an excellent experience (flying Emirates is another choice that breaks the trip up via Dubai). You still go to Singapore, then do a Singapore-Doha (in Qatar, in the middle east) flight of about 7hrs, then a transit stop in Doha airport where there is an indoor playground perfect for small children, for about 3 hrs as I recall. Then another flight of 7.5hrs to London. Best thing was, they slept on the Singapore-Doha flight, got woken up and fed, had a few hrs to charge around Doha airport then another flight that was actually managable. On arrival in London they went to bed around 5pm and woke at 3am, got fed and got told to go back to bed - as a previous respondant commented, NO lights, NO TV, minimal lighting and disturbance! They didn't really sleep again but rested in bed, then got up around 6am. We kept them up until 6.30pm that night, they woke around 5am the next morning but stayed in bed, and after that were pretty well over jetlag. I'm pretty sure that the broken flight really helped.
If you want to try things like Phenergan to establish abit of sleep I'd definitely try it at home first. Some kids react the other way and actually get hyped up - this happened to a friend who gave some to her 2yo for the first time ON a long haul flight - both she and the other passengers had the flight from hell!
Anyway, good luck with your trip - it's all worth it even though the actual organisation and planning can drive you crazy initially!