I've got to second JJ's suggestion. I spent most of last summer in Brittany. Hardly saw any other tourists, let alone US tourists. The beaches were spectacular and the coastal paths breathtaking. The only drawback is that sometimes it is difficult to get to the out of the way places on public transportation. That's why I elected to go with a motorhome.
If you do decide to spend more time in the UK and not leave for the continent from London, there is a ferry from Plymouth to Roscoff which puts you in the west of France. It gives you a chance to see Brittany and work your way back east. Whatever you do, don't miss Mont St. Michel. It will be crowded with tourists but well worth it. There is a very nice campsite in the town about a half mile directly south of the Mt.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13