It can only really be the palace or number 10
Or plain incompetence / miscommunication (although why you would hold them for so long after the ceremony is not clear to me – other than to try and cover up your own errors).
Or Special Branch / MI5 don’t talk to the right people in the Met.
Or the home secretary.
Or more likely a load of “group think” from officers who were poorly briefed. At the end of the day you have a structure where officers need to be empowered to make decisions on the ground and act quickly (imagine if they had been going to do something much more disruptive or dangerous and the cops stood by waiting for instructions if these were the right sort of bad people to arrest). What matters is that having made an initial arrest there are checks and balances to liberate people as soon as possible. Unfortunately all too often problems arise not from the original mistake but the delays in correcting that mistake.
I raised a quizzacle eyebrow when the Police Federation came out yesterday to say that the arrests were entirely legitimate and appropriate. It struck me as entirely not the job of what is essentially the Police Union to be commenting on whether arrests were legal or not, it already seemed to me that someone was looking for individual officers to blame. I’m certain that this will become the Home Office position – individuals got carried away rather than new the law being shit or the culture being one of cover-ups and lack of oversight.