I think an interesting question would be what would your response be if the radio isn't turned off. Quite obviously you can't just say, "help I've got a gun to my head" if the man behind the gun has told you not to... and your worried about getting shot at the end...
In the US of course we have a squawk code and I've been advised by my local tower that any combination of that code in my normal radio call would be picked up on, ie, changing my N number, advising crossing the runway 7600 etc etc...
Not even sure what the UK equivilant to that squawk is...