The actual message itself was recorded by the chief continuity announcer of BBC Radio 4, Peter Donaldson in the 1980's. It was accompianied by "Dalek" music and strong pulses of light. The existence of this warning message was officially confirmed by the British government on New Years Day 2006, even though it was an open secret in the BBC.
In the 1962 film 'The Hole In The Ground,' the attack warning is given by the BBC from a record kept in a locked safe - 'this is an emergency announcement. An air attack is approaching this country. Take cover at once!'
I think it was alsoi parodied by Frankie Goes to Hollywood in the song Two Tribes, as 'The air attack warning sounds like this. This is the sound. When you hear the air attack warning. You and your family must take cover'