イギリス側が考えているのはこういうことのようだ

For the United Kingdom, and BAE Systems, in particular, having an
international partner on the Tempest program should bring down
costs and reduce overall risk, especially as Japan would likely join with
a firm commitment to buy aircraft at the end of the development period.

詳しくは
"Did America Just Miss Its Chance To Help Build Japan’s Future Fighter?"
Reports suggest that Japan may well join the British Team Tempest
future fighter program by year-end.
BY
THOMAS NEWDICK
MAY 17, 2022 10:58 AM