We have a Modulator in a facility that whilst I've lived in this are for 20+ years I have no idea what it is. I do believe however it is military:
http://goo.gl/maps/gU3g5 Signal unknown.
Another system is a Radioactivity leak warning system at HM Naval Base in Portsmouth due to it being home to the Royal Navy's Nuclear Submarines. It is tested every few months and consists of at least four Carter/Castle Castings sirens which sounds in attack and scares the crap out of people:
https://www.youtube.com/watch?v=ra1T1nypBDA
I have also read that there is one the other side of the harbour in the RNAD (Royal Naval Ammunitions Dump but (quote) "there isn't anything nuclear there...at all..."
There is also a similar siren at an Ammunitions port in Southampton that is sounded when live ammo is moving in/out/around the port. Attack signal.