Japanese Fashion? Not at All. This is "True Brit" Design by Kim Jones.
According to The Wiks, KJ is the "designer behind David Beckham's morning suit at Prince Harry's wedding in Windsor". I think "behind DB's suit" means he designed it, rather than anything physical.
Following El Reg's links leads to Uniqlo's site, where スリムテーパードパンツ(カラーブロック)(KJ) [Slim tapered pants (colour block) (KJ)] is offered for ¥999, which is less than a tenner. Lower down, you can see that these pants are part of Kim Jones' collection for GU. The "colour block" is simply a zip fly cover flap. Even if you wanted to, I do not think it would be physically possible to use this flap as a kind of sheath. I would not want to, and do not find the colour contrast attractive, but some people might.