From the Flightline UK news entry:
At the recent DASC Pre-season flying display symposium, it was announced that all flare releases have been outlawed. This decision was made as the manufacturers of the flare systems could not be certain that no debris would reach the ground following release. Any such debris could prove to be harzardous to persons and aircraft below.
Some of the most prominant users of flares at airshow have been written to by the UK MoD asking them not to employ flares at UK shows. At the symposium, it was also confirmed that this regulation affected the coastal airshows as well as those held over airfields.
Last year the Dutch F-16 demo was not allowed to drop flares at the Volkel Open Days for the same reason.
The team's crewchief told me that the flares used by the F-16 don't burn completely, and some residue (small metal plates) falls to the ground.
This could in turn result in FOD when it ends up on a runway or taxiway, or cause fire when it falls on dry grass.
The Apache doesn't have this problem, since its flares burn hotter, so the metal ignition plates burn up as well.
He told me that the flares are interchangeable, but they aren't allowed to use the Apache's flares on the F-16, since it isn't tested and may cause problems.