[Excessum]

Booking trips is always a game between availability and affordability and if you want to get the best deal, you have to wait for the best moment to book.
As a former travel agent, I know that the sweet spot for booking airline tickets and most hotels is 10-13 weeks in advance... But you always have to keep the seasonal differences in mind. For Japan it is always hard to get good deals during most of the spring (The sakura blossoming season, which is the definite high-season for Japan), but there should be no problems with summer.
I would post-pone the booking of tickets/hotel till April/May if I was you. Check the prices during April couple of times a week, they will go down while the availability should still be pretty good during the month and then book 'em at the late May.
And you can always give the airline a call directly and ask for availability during the time of travel, so you can play it safe... same as with the hotel(s).
If, however, you have no problems with spending the above mentioned sum of money for the trip, then go ahead and purchase it now, but if you feel like cutting the price down by a couple of hundred of quid, patience is your best friend =)