It's not an impossible way to go about it. I'd just recommend that you keep two things in mind, first, that if a public diplomatic incident is the thing that kicks of the escalation, surprise will be a lot harder to achieve. Second is that it is impossible for the Japanese to pull off a Pearl Harbor on the British. Not because of some sense of RN superiority but rather due to basing positioning. The only area that the Japanese could acquire total surprise at is Hong Kong, which if the RN commander has half a brain will not be where the fleet is stationed.I’m still open to what occurs. What I have planned is the Japanese accidentally a public diplomatic incident, and don’t back down and apologize. Britain moves a portion of its fleet to the Pacific to keep an eye on Japan and to remind them who has been a naval power for longer but this causes Japan to feel cornered so they launch a surprise attack to show the world they are to be taken seriously.
That’s my placeholder idea so I’m open to ideas. I just thought a Pacific War starting in 1940 to be interesting.
If necessary I’ll push it back to early 1941 to give the Japanese more time to prepare. It’s a work in progress.
The most likely place for the fleet would be Singapore, which if the French don't hand over Vietnam to the Japanese and the Japanese do not take the Philippines, would be incredibly hard to get to without being detected.
Hong Kong is fucked though.
Regarding the IJN, since you can't exactly rush shipbuilding, a 1940 war will mean they start it without the Yamato, nor the Shokaku and Zuikaku aircraft carriers. Considering they were part of a shipbuilding schedule, you'd probably need to change that schedule well back into the early 30s if not late 20s.
If you want to start a war in 1940 with Japan, the absolute best way to do it would be to get pride involved so that they go in ill-prepared but unwilling to back down.