Parece que los japoneses temen retrasos por parte de los británicos, con problemas presupuestarios a corto plazo: https://www.ft.com/content/c3de1d53-4aa8-4e11-87b1-409172bdc3ef?syn-25a6b1a6=1
"Tokyo’s main aim is to produce a new fighter by the mid-2030s, whereas London and Rome are more interested in GCAP producing a cutting-edge “system of systems” — a jet that operates in concert with a swarm of drones — over a less rigid timetable, one of the people said."
"One of them added that this could leave the UK “tempted” to try to slow the timetable to make it more affordable in the short term."
...
"Tokyo has shifted its stance to become more open to signing deals with potential customers for the fighter jet to reduce the funding requirements on the UK, with Canada a leading potential customer, according to two of the people."
...
"Japanese officials stressed that Tokyo is reluctant to allow the entry of any new design and development partner, however, given the complexities. Japan resisted a push last year by the UK and Italy to get Saudi Arabia involved in the development. Saudi Arabia was still in talks to join in some form, according to two people familiar with the situation."