Major plot hole

I wouldn’t say major plot hole. The movie doesn’t say how accessible / non-accessible the underground lair was. I don’t remember when they said his lair was connected to the prison where he was incarcerated.

They know nothing about him. The masters of Larkhill don't realise who it is until it's too late, and they were "in the know". This version of England would have SO MANY dissidents and rebels that process of elimination wouldn't work. The Underground is likely whorls of labyrinthine tracks used and unused and a search would be mind-numbing. You're not going to commit your whole police force to just go down into the sewers (this isn't The Dark Knight Rises) until you know more.

The lair was likely secluded and camouflaged by V.

To be fair, that very plotpoint came from the Graphic Novel this is based off of. It's revealed towards the end of the movie after Evey is tortured by V that the only way out and in is either through an elevator or some subway tracks. So, yeah. Nobody would think to look in those places.

Th only hole was in your head. It wasn't connected to his previous prison. The prison you see Evey in is a fake one created for her by V. Did you eve watch this movie or just skim through?

If I recall, in the film, Larkhill was described as being 10 km north of Salisbury.

I believe the "Shadow Gallery" may have been an annexed or administrative building near the Old Bailey Courthouse. It may have held lawyers' offices, other legal/police offices and had prisoner holding cells for those awaiting trial. Prisoners would have been transferred to and from Court via tunnels rather than using unsafe/unsecured streets. The building may have been "rustic" or "heritage" distinguished but vacant due to developers not being allowed to destroy/demolish a historical site.

V for Vendetta 2 -the rEViVal EVolVes (fan fiction)
http://members.distributel.net/~artemis/the_rEViVal_EVolVes.html
V2 through V5 : http://members.distributel.net/~artemis/V-dom_for_Freedom.html

Not a plot hole as others have indicated. His lair was in London, Larkhill is like 140 kilometers (80 miles) from London.

The prison part was recreated by V. I'm not entirely sure if that's mentioned in the movie, but it is in the graphic novel.