Originally Posted by CPH-Flyer

Generally, JAL has no problems with handling that for you. JAL has through check across separate tickets within oneworld as a stated benefit. The issue could be that you are not eligible to enter Japan, and JAL in BKK would see you as terminating in Tokyo. But people have not had issues with that in trips similar to yours via Tokyo.

Provided your connection is less than 24 hours. Make sure you have all of your entry requirements for UK fixed, and have your e tickets printed for ease of showing the details.