あと、その前にこう説明してる

Sato said that the collaborative programme – named the ‘joint demonstrator engine' project – aims to prove capabilities including power generation and thermal energy management in an engine that is sufficiently compact for the GCAP fighter.

“We are now sending IHI engineers to Rolls-Royce and they are sending engineers to us in Japan,” said Sato. “There are a lot of new technologies that need to be validated. Another priority is the design of the engine to ensure it can be integrated into the new fighter aircraft.”

メーカーの説明は明確に「新技術を持ち寄った共同開発エンジン」だから
日本単独開発の妄想は隔離スレでやれ