Apart from sandy beaches the W coast is pretty dull until you get north of the Loire. The only real towns of interest (IMHO) are Biarritz, Bordeaux and La Rochelle. My preference would be to head north from Millau to play in the Massif Central and then if you really want to head for the coast you could follow the Loire (roughly) to Brittany.
If you are after submarine pens you can see them at St Nazaire where there are also various museums and a submarine you can visit. If you speak French you can go on a tour of the dock yard. I was really lucky, there was marine architect on my tour and between him and the guide it was really informative. The Queen Mary 2 was almost finished when I was there.
The pens at Lorient are still used as a submarine base and for building minesweepers. I'm not sure that nuclear subs would fit. When I visited (in the late 70's) you had to show a French ID to go on the tour. Don't know what the situation is now but of the 2 I found Lorient more impressive inside. You get a better outside view at St Nazaire.
Hope this helps.