The more I think about it, the more I think there is no H and if there is, it does not have to be Buckells.
Although Osborne is bent, I don't necessarily think he's either H or the big baddie pulling the strings. If he is H then Carmichael is clearly complicit in covering up for him. Does that make her bent? Probably.
Personally I think there IS someone else pulling the strings and that person is outside of the force, within an OCG. The actions of the OCGs themselves are not within the remit of AC as AC is only concerned with bent coppers.
I think the "moral" of the series is that corruption is a bit like a hydra - you chop off a head and another grows back. it can't be stopped. Bentness can be overt - like Buckells - or less overt - like Steve and his inability to keep it in his pants, Kate and the affair with a suspect's husband. Is Ted bent because he gave dirty money to Steph? Yes, technically. Is Osborne more bent than Ted because of the cover-up of the shooting? Was Maneet bent for covering for her dim cousin and getting that information for Hilton? Yes - but she still died in the Line of Duty nonetheless (as I recall).
So everyone is either bent in some way or corruptible. For me at least, bentness is not about big baddies and massive reveals - it's about the subtler, institututionalised, everyday stuff that will always go on whilst there are people within the force willing to accept it and self-interested enough to maintain the status quo.
So no matter how much you want to stop corruption, it's always going to be a losing battle and Ted "the battle" Hastings has lost! He's out, AC12 is no more, Carmichael is in (for now) and Osborne will probably retire before any disciplinary action is brought against him.
Poignant, really.