From the Good Law Project (reasons for MET police not investigating) :-
First, they say they relied on the Government’s assurances that no rules had been broken. Then, they say there would have been no point in interviewing No 10 staff about the parties because they would have refused to answer questions that exposed them to a risk of prosecution.
In what other crime would police decline to investigate because the suspect assured them no rules had been broken? (or because they would refuse to answer police questions – comment by Hymie) And those justifications can’t both be true; if no rules were broken, there’s no risk of self-incrimination.
For more information see here
https://www.crowdjustice.com/case/met-police-investigate/?utm_source=CrowdJustice&utm_campaign=9784045eed-10PARTYJAN2021&utm_medium=email&utm_term=0_7304756a43-9784045eed-169393273
The Good Law Project are issuing formal legal proceedings to force the Met to revisit their decision not in investigate the criminality at 10 Downing Street.