In 2024, I made a trip to Japan. Flew Japan Airlines straight from San Francisco. In the planning process I became aware that I could've gone via Europe/ England etc. The idea of adding to the trip with a stop in the UK occurred. Would there be any tricky visa requirements or other special hazards involved if one were to fly via London Hethrow from the USA to the orient ?⁹
You could, but why would you?
You could go via Australia, Africa, or probably just about anywhere else if you choose to do so. It might cost a lot more and take longer, but it's still an option.
As for Heathrow, it must be 20 years since I had to endure that experience. There are good reasons why I have avoided it. I know it was a long time ago, but from what I hear, if anything it has only got worse in the interim period.
If you must visit UK (again, why would you?), or Europe, I wouldn't have thought that trying to incorporate it into a trip to Japan from USA is the best plan, but it's not me who's putting the idea forward. Your cash, your time, only you can decide.