It's a bit of a grey area, both legally and practically.
Many pilots file an airport as an alternate which needs PPR, or PNR/Customs, without obtaining the PPR/PNR.
Within the UK it doesn't really matter (unless you pick LGW which being 24hrs PPR might get "interesting", short of declaring a mayday) but in some places abroad one needs to be more careful.
The bottom line is that if you arrive at your filed alternate, and they refuse a landing, then you really do have a 100% legitimate Mayday situation, which means they must allow the landing.