Matt, France is easy from Plymouth. About a 7 hour trip across to Roscoff. I have done a few trips across and a couple of times have gone through France to Switzerland, then back into France, over the Pyrenees to Spain and caught the ferry from Santander back to Plymouth.
Brittany ferries is the company if you ever think of trying it. A trip like that above would cost about £350 if you have a cabin each way like I do.About £100 less if no cabin.