Well, this at least solves the conundrum. They obviously did have a party. Then, they childishly chose to deny it, laughing as they did so. The bigwigs, including the Prime Minister who was upstairs, presumably all knew perfectly well that it had happened and the Chief of Police decided to be lenient about it. It puts them all in a big hole for a monster cover-up.

https://www.itv.com/news/2021-12-07/no-10-staff-joke-in-leaked-recording-about-christmas-party-they-later-denied