Longhaul, my only experience with Christmas flights is to Japan. In order to make the most out my holidays (and spend as many days as possible with my then fiance) I was booking my flights my March/April, even by May or June the prices would have already started to climb. Nearer the time, they just went up to ridiculous levels.

You might find some bargains for around the 17th, 18th December (but looking at the award flights to Toronto and North America for this time last Christmas I wouldn't count on it). You're more likely to get good prices for dates before the 14th December - and that's likely to be too soon. Same for travel back, for a week or so after Jan 1st flights are very popular. Between Christmas and New Years Eve there is the occasional bargain.

From a Brit's point of view (esp. one who has to pay the flight prices for travel to Japan) 425GBP doesn't seem too bad. I know full well it can go much higher than that.

Perhaps one option is to pay this and get the most popular dates possible (i.e. 21st or 22nd December) and put yourself forward as a candidate for voluntary bumping. I'm sure others here can advice you on the best airline and the best tactics to use if you decide to try and bring the costs down via that route.

It's just that I've never, not once, seen costs dip nearer the winter holidays, only the opposite.