My wife (of three years) and I had recently vacated RAF married quarters on my discharge and were in our new (3-room) apartment, watching TV, when the regular broadcast was interrupted and a test card PLEASE STAND BY appeared. I think it was about 18.30 Zulu (6.30pm Greenwich Mean Time). Five minutes later a news announcer came on to say that someone had shot at the president during "what Americans call a motorcade." After promising to keep us up to date, the regular program continued.
About 45 minutes later the show abruptly switched to the news desk, where the announcer said that unconfirmed reports were that President Kennedy was dead. 20 minutes later that was confirmed, the newsreader saying that the biggest manhunt in American history was under way. He went on to say that, in view of the tragic nature of the event, regular programming would be suspended. Test card again. Finally, for the rest of the evening, there was just somber music and some videos and stills of Kennedy. Of course, this was long before satellites, so there were no live shots or film from the US.
At 11.30pm there was a brief news bulletin with an update about a suspect having been captured, and then BBC TV shut down. I was in Cat. E of the reserve (subject to recall in a national emergency), and received a phone call to stay by the phone until a stand-down and be ready to report to the nearest base if instructed (there was fear that the assassination might be the initial act in a nuclear attack).
The next day was somewhat similar, with frequent interruptions. The comedy/satire program That Was the Week That Was, starring David Frost, was due to broadcast, and we all wondered if the BBC would cancel it in deference to the feelings of Americans in Britain. When the show started on schedule, a lot of us thought "how crass!" but instead, a completely different, shortened 20-minute version of the show aired, something that is still discussed by those who saw it. No comedy, no sketches or satire; Frost and the others speaking about Kennedy. A song In the Summer of His Years," lyrics and music composed in record time, sung by cast songstress Millicent Martin, ended the program. Americans, particularly in the US embassy, were so moved that a videotape of the program was flown to the States and screened by NBC the next day, and ABC later. Decca Records released a disk of the song, which was also later sung by Connie Francis, Kate Smith, Sarah Vaughan and others. The Francis recording became a Top-40 hit.
Some commentators in Europe were surprised that the British made such a big thing about a US president who was not really close to them - certainly not as, say, FDR had been in WWII - and wondered if America would have done anything close to this if, for example, the prime minister had been assassinated.
Speaking of WWII, there was quite a lot of bitterness in Britain a couple of years later when ex-president Dwight Eisenhower didn't bother to attend Winston Churchill's funeral in Westminster Abbey. This surprised and offended many, seeing as how, as Supreme Commander, Allied Forces, Europe, he had worked closely with Churchill (who was commander-in-chief of all British forces), and also interacted with him during the time in 1953-55 when Ike was president and Churchill prime minister.