The actor who played Buckells has just been interviewed on BBC breakfast.
He was asked about the smirk Buckells gave as they locked him up and said it was because he knows that he won't be inside for long, and it will be brushed under the carpet, due to the focus being taken away from "a few bad apples" by Osborne, Carmichael etc. Literally scaling back AC12 and like it said at the captions at the end, currently AC12s powers have never been weaker.
He said he got a text from Jed during first lockdown when they suspended filming and had only seen 3 scripts, telling him that he was H (or the 4th man) and the cast were then sent the last 4 scripts, so they've all known and kept this under wraps for a year before filming recommenced. He told his wife, who was sworn to secrecy.
He also said he totally understood that some viewers would feel initially disappointed with the final episode, but hoped that when they reflected on it, they realised that the main message wasn't about finding out who H was - but how the whole structure of uncovering police corruption is impossible and is being blocked. Hence the loose ends, and Ted's utter frustration at Carmichaels new set up.
He said some viewers felt cheated perhaps because they wanted the police shoot out, "Urgent Exit Required" ending but real life and policing isn't like that - and this series reflected the reality of that.