So I’ve been working on this theory for a while now. This is a little long, so please bear with me.

TLDR; Inspector Finch’s lieutenant Dominic Stone was working with V the entire time as a mole, feeding him information to succeed in his plan.

So at the beginning of the movie, we have some large coincidences that are either plot holes or crazy coincidences.

When Inspector Finch (hereafter “Finch”) & Lieutenant Stone (hereafter “Stone”) go to the BTN tower to look for Evey, the only one with a legit shot at finding her is Stone. He actually enters the same room as her and takes a cursory search around knowing full well someone hiding from authorities for terrorist activities might not just stand up in the center of the room. You don’t make Lieutenant that way.

Furthermore, the only person who had a proper opportunity to kill V early doesn’t do it. And V, instead of slashing his throat like he did to 10 other redshirts, spares his life by just knocking him out.

In probably the best evidence for my theory in the film is during the scene where Evey is looking for a way out of the Shadow Gallery and asks V to help with his plan. In the meantime, Finch & Stone start to narrow in on Lark Hill and it’s potential relationship with V. V then comes to Evey and tells her that due to “unforeseen circumstances,” he has to accelerate his plan.

V doesn’t seem like a guy to have many things not planned out, so he must know his secret is almost out and when it is, they will protect the other high level party officials so heavily he might never get to take his shot. Since Stone is informing him of Finch’s progress, it’s clear that this would immediately follow the first time Finch brought up Lark Hill to Stone.

After this incident, Creedy is forced to takeover the investigation by Sutler. Creedy informs Finch that Sutler believes V may have an inside, a theory which Finch believes is Creedy is calling him treasonous - but is actually a perfectly reasonable argument for how V is always one step ahead of Finch.

Once Evey escapes to Deitrich’s house, we find a lot more evidence that V is carefully listening in on conversations of the police. How? Because he rescues her from Creedy’s black-baggers outside in full riot gear dressed as a member of the SWAT team.

He can’t simply be walking around every night, following around the SWAT team - so he knew they were going to Deitrich’s house and was worried they’ve find Evey there. How did he know? Because he had been watching Deitrich knowing that Evey and he were close. So once the show aired and they received word at HQ, Stone told V what was happening and he decided to try and rescue her.

A bunch of plot holes exist here if he isn’t colluding with Stone. Did he regularly go into raids with the black-baggers? How many did he rescue while doing that, or did he just hang out with them, let them bag, and sneak off after? How long must he have been doing this for - and how often, considering Creedy tells Sutler that arrests are as highest as they’ve been since the Reclamation.

After this, when we see Evey again later in the film, V tells Evey that Deitrich was captured but that things really got bad “after they found a Quran in his house.” How would he know they found a Quran there? Someone is leaking to him...

My last piece of evidence is when Stone goes into Finch’s office to tell him about a document he found that had 3 names on it - 2 were people V already killed, and the only one left was William Rockwood. That action caused Finch to open the email he had ignored, which then lead to him stalking Creedy, which of course ends with Creedy dead like V wanted.

If Stone hadn’t gone in there with that document, Finch would have never read V’s email and he wouldn’t have gotten to set in motion the Creedy part.

Now you could argue that he clearly had some hacking capabilities to be able to hack the audio system early in the movie to play his symphony while blowing up the Old Bailey. I considered this too. It’s dismissed, though, by the idea that some of the info is coming in in REAL TIME as things are going down, and haven’t been transcribed to data yet.