Do you have a source for that last example? Seems like it would be a bit difficult to keep a tunnel of that scale secret.
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.