Rudloe Manor could be considered as the UKs' A-51. Not on the same scale though because its in a residential area

I visited it a few times, the
whole area is littered with ventilation shafts for the underground tunnels, and there is real tight security with MoD police over the place. I think
there is an article on ATS about it. You really need to go there to really see the scale of the tunnels.