The only "proper" cqb site I can think of is Red 1 in King's Langley. Alternatively there's Apocalypse near Royston, while it's a woodland site the gamezone is quite small with relatively short engagement ranges
Sentinel in Dunstable as mentioned above is fighting around a singular building that has a medium sized killhouse type layout inside
Best bet is jumping on the A1 and going to The Gaol, I'm in Knebworth and thats my go to for CQB. If you dont mind outdoor games another go to of mine is Reforger which coincidently is almost opposite Red1 - Large woodland site with a built up "village area" and multiple long games played through the day over fairly large playing area's.