"I suppose it'd be rude to ask if you've got any facts to back it up?"
Look at the history of the last 50 years - - almost all serious European military hardware has been produced by consortiums of countries and several of those failed because of costs/work share issues
The UK and Italy are middling economic entities - Japan is richer but has rarely participated in multi- national developments. The estimated costs are already way north of £ 25 Bn - up to £ 40 Bn - which is way above any indicative long term funding. We can't even afford to buy F-35's at a reasonable rate - how do you think we can afford a completely new aircraft?
look at the comments here
https://ukdefencejournal.org.uk/temp...jet-programme/
The current £ 2Bn is really to keep the design teams at BAe etc ticking over