any thoughts on buying tickets from Japan?

I am looking at leaving Sydney in May and travelling to Tokyo/Kyoto for say 10 days.

Mostly then spending 6-8 months in UK, including some short side trips to Europe (that we could buy there).

Then to New York, San Francisco (possibly San Diego or Portland to visit friends) before returning back to Sydney in December or January, if possible via Honolulu or similar relaxing stopover

2 adults, a 4 year old and an infant who will turn 2 in September while we are travelling.

Fares from Syd are generally so much more expensive than elsewhere that I wouldn't mind getting into a new cycle buying fix from UK as I used to when I first moved out here..

Thanks