How can ATC clear you for a PRNAV procedure if they don't know your equipment fit ?
That's why the system takes it from your flight plan.
I didn't mention anything about mandatory airspace or your plan being rejected by CFMU (although I think you probably mean IFPS), simply that the lack of PRNAV is flagged up to UK ACCs. This is so they can give an appropriate clearance to an individual aircraft ..... where it's appropriate for the filed flight and routes exist. If you have the choice of following a PRNAV route or a 'normal' one, then at least UK ACC's are going to know which one you can accept without getting in to a long winded conversation on the RT about what you can and can't fly.
Assuming you've filed the plan correctly in the first place of course
