[Alex]

If you are not afraid of making a trip through France (and I mean yes, you can be afraid, most of us still not speaking English), I'd tend to say the best base to visit for you would be St Nazaire.
It's the one I like least, since a part of it got completly "freshed up" (part of walls demolished, little shops created inside the bunker) but at least you can get full access to it, including the roof.

The only problem I have with this one is that it's no more the original one as it was during the war.

Other than that, don't expect to be able to visit others. The Brest bunker is a military base of some military section of the navy, Lorient bunkers are the home of some companies related to maritime construction, La Rochelle is no more accessible to the public due to risks of roof collapse rising recently, Bordeaux got closed to the public for some reason related to the protection of its inner walls (a bunch of idiots came in and got some walls graffiti-scabbed, therefore the mayor of the city decided that was enough to close it indefinitely).
But still, you can take pictures of absolutely everyone from outside of them if you like.

Have a good journey !