u/MR_Happy2008
The UK's Nuclear warning systems
There was two system that was to Warn the UK in the event of a nuclear attack was confirmed by RAF Fylingdales or the USA's Defense Support Program
HANDLE which was a network of 252 carrier control points fitted in police stations linked to thousands of Receivers across the country in Police, Fire Coastguard stations, Government and Industrial buildings, Hospitals and Royal Observer corp's observation posts some in rural areas were installed in post offices, public or private homes and churches (example: https://youtu.be/PBQfRv8HDdQ)
4-Minute warning was the recorded message to be transmitted by the BBC in the event of nuclear war it would also work along the BBCs Wartime Broadcasting service in 1991 this according to Wikipedia was the message "Here is an emergency announcement. An air attack is approaching this country. Go to shelter or take cover immediately." Repeated twelve times across 2 ½ minutes a couple other versions exist.
1971 UKWMO film 'sound the alarm' which reads "Here is an emergency announcement. An air attack is approaching this country now. Go to shelter or take cover at once."
1979 Government training which reads "Attack Warning Red. Attack Warning Red. An air attack is approaching this country now. Go to your refuge rooms immediately and remain there until you are told you may come out." If there would have been used is unknown.
A network of sirens would have been activated most dating to world war 2 or newly built for the cold war (example https://youtu.be/ERE8RzqyVfE) along with other forms of communication being used if unavailable like whistles
Following the attack the BBC would remain on the air if possible using the Wartime Broadcasting service which would broadcast news, information and music to try and maintain morale even as most of the country would be destroyed or dying of radiation
(Example: https://youtu.be/bcWpGwn8IDY).
Sources and Further information:
Wikipedia https://en.wikipedia.org/wiki/Four-minute\_warning
https://en.wikipedia.org/wiki/Handel\_%28warning\_system%29
https://en.wikipedia.org/wiki/Wartime\_Broadcasting\_Service
Period Films produced by the UKWMO / ROC which shows the system of warning and monitoring posts that would produce and provide fallout and radiation information:
u/MR_Happy2008 — 3 days ago