[ViridianFan]

As mentioned the health of the royal family is a never mad en public so that announcement at noon was (for want of a better way of putting this) a warning to be prepared. 

I know someone who was involved in operation London Bridge and they had been initially told an announcement would be made at 1800. I know for the ringing of church bells the cut off was 1800 for ringing that evening, and I believe that it was also the cut off point for it being D day. Id need to dig out all the planning we had for ringing however it being at 1830 a lot of places did D day events. 

An article I read by an ex bbc producers said it would be likely that the bbc and other broadcasters would have made aware prior to the official announcement from Buckingham Palace, if this happened or not I’m not sure. I believe the PM was informed just after 1615.