If this kind of thing interests you and you are in the south of England (or even Brittany in France) you should definitely check out the WW2 German bunkers in the Channel Islands. They are easy to reach by sea or air, especially during summer. I'm in Jersey and have access to a few large ones, contact info is on my profile page. Here is some background info if you are unfamiliar: https://en.wikipedia.org/wiki/German_occupation_of_the_Chann...
We regularly have people visit from all the way up and down Hitler's 'Atlantic Wall' and they really like the number and condition of the bunkers here.