Maybe a good place to fire off old flares might be near Military Ranges. they use the same stuff. Your old flares would just be assumed to be squaddies training.
My view is its always a shame to waste anything you've paid good money for and by firing off your old stuff you will at least gain some experience from it.
Now how you manage to do that without alerting everybody is another one.
Personally I reported someone to the Police a few years ago because a para flare was fired in the fields behind my house. I knew what it was straight away when it was fired because of my own experiences. It certainly wasn't a firework. And as such there was no excuse for firing it. The top end of Hampshire is just farming lands and towns for those that don't know. No water and no mountains.
The Police were not one bit interested. (Whats new).
Seems like 5th Novemebr is the best time to fire off any old Pyrotechnics.
God Save any Sailor that gets in trouble on that night. In fact thinking about it Guy Fawkes Night is probably the worst night of the year to go sailing