A couple spring to mind, one I actually managed to explore and one I missed out on by a good seven years but when attempting to dig up info on the first stumbled across by accident.
The one I missed was a very mysterious, quiet place simply called 'MOD Aquila' - as the name suggests, a Ministry of Defence site, specifically a secret research and development complex. There is almost zero information about what the purpose of the complex was, and at the time no photos at all. The well known explorer Simon Cornwell managed to get inside (whether it was permission or not I don't know) shortly before the buildings were demolished in 2004, and these are the only photos around of the site available to view...mysterious indeed.
http://www.simonco.../e020304/index.htm The mysterious location I actually managed to explore was another military site which also had almost no readily available information about it's purpose on the net, most of what I know about the site I gleaned from bits and pieces seen in various buildings, and from paperwork rescued whilst the buildings were pretty much being demolished around us at a later date. It was located about two miles from one of the UK's best-known locations, the National Gas Turbine Establishment (or Pyestock as it was better known), closed at the same time as it in or around 2000 but laid undiscovered until a decade later when bits of it were starting to be demolished. It was called the Centre for Human Sciences and was an amalgamation of various military and medical organisations run under the Defence Evaluation and Research Agency banner. In the huge facility they tested and developed equipment for use by military personnel engaged in combat situations at home and abroad - some of the facilities included a pair of enormous climatic cold chambers and matching hot chambers to test extremes of temperature on equipment and the people using the equipment, as well as hypobaric chambers, anechoic chambers, fighter plane cockpit testing rigs as well as the UK's only fully operational human centrifuge which was the only active building left on site.
I explored the site 5 times between the end of 2010 and beginning of 2011, the site is totally demolished now apart from the operational Centrifuge. In my eyes it goes down as one of the most overlooked locations ever to pop onto the scene given it's proximity to perhaps the most famous urbex site in the UK
Here are some sets from my times there...
https://www.flickr...72157625458879205/ https://www.flickr...72157625722221323/ https://www.flickr...72157625848372338/