They have an F-35, and they're already building a 6th generation fighter. GCAP got billions for development

The 6th generation GCAP (Global Combat Air Programme) fighter construction program entered the next stage. The United Kingdom, Japan and Italy have granted a £4.6 billion contract (approximately €4.4 billion) for further improvement of the programme and the fresh combat aircraft to enter service from 2035. Importantly, all 3 countries already have V generation F-35 fighters.