There is a tunnel in Aber that was used to store valuables (Crown jewels, etc) during WWII, but it's certainly not a tunnel all the way from London.
Source: http://www.everythingaberystwyth.co.uk/aberystwyth-fact-of-t...
The bunker in question isn't on any of the lists you'll find on the Internet. The bunker is also very well hidden. I can't find the entrance on Google Maps, but I'm very sure of the location within a few kms.
When people purchased bunkers post-cold-war, they were confronted with "permanent sealed" doors that cannot be opened within the bunkers. These are welded shut and the ports on the door filled with concrete so that it filled whatever was behind. You can go and see these at Kelvedon Hatch in Essex and could previously see these at Ash Radar Station in Kent (before thebunker.net bought it). Each of these doors is closest to the "London side" of things. When they were built, people spoke of the ground vibrating for months at a time in various places along the routes apparently as well (although I can't find a source for that -it might be well just be rumor).
They would be very small tunnels (pedestrian/small vehicle) sort of like cheesy James Bond bad guy monorail sized outfits but it's not beyond the realms of possibility.
Those who worked on them though are sworn to secrecy or are dead so we'll probably never know. That is until someone has the balls to fight their way through the welded and filled doors which would take weeks and be extortionately expensive due to the diamond belt saws needed I've been assured.
Since someone reading this might have similar interests, I have a question. I have recently seen a document, which I think must have been part of the Snowden leak, but I'm struggling to find it again. It s a map of all of the listening points from the Tempora program. They were all common first names, differently sized boxes on the map.
If anyone knows what I'm talking about then I'd love a copy.
^^ start here. I'm sure there are maps somewhere.