From Alsace to Brittany

Hop! (Air France) has 1 direct flight from Strasbourg to Rennes on Sun, Mon, Wed, Fri.

Where do you have to be at the end of your Breton sojourn?

Options are: 1. flight to Rennes or Vannes (if they fit your schedule) and rent the car there; 2. train to Dinan or Vannes and rent car there; 3. rent car in Alsace and drive to Brittany but it's too far for one day, you'd need to overnight somewhere. How long do you have to get to Brittany? Where in Alsace will you be coming from? Not sure one base would suit for all the places you want to see in 4 days. If you go from Alsace to Dinan, which is the one location you name that is in NW Brittany, you may want to spend 1 night there and then move to a base that would make more sense for your other locations, which are more S/SW Brittany.

I will go back to London at the end of this trip.

There is one direct Strasbourg-->Rennes high-speed train a day, leaving Strasbourg at 8:01, arriving Rennes at 12:17. I've taken it.

All others you have to change once, and you don't want that change to be from one train station in Paris to another if you can avoid it.

We have driven around that area twice. We got our car in Rennes at the train station. A great book to help guide your road trip is Frommer's? 25 Great Drives in France (Best Loved Driving Tours) . We have used it to help us plan and then go from there. We usually don't spend more than 2 nights in a place, often just one, because everything in Brittany is so close. Just drive from a small town to another. Hope this helps.

As others mentioned, you could take a train or a plane from Strasbourg to Rennes. According to places you would like to see, Vannes or Josselin could be good bases (Josselin is small but pretty). Once in Rennes, you could rent a car, or take another train to Vannes.