Brittany in September with No Car; Dinard Airport Jaunt to London??

Frankly, if it was me, I would not do the jaunt to London. Too many things can go wrong, from Icelandic volcano ash to a labor strike to a terrorist scare.

Looks like you have researched Brittany very thoroughly. You'll have a wonderful stay, I'm sure!

There's no bus taking you to Dinard airport, you need to take a taxi if you don't rent a car (about 10€ from Dinard). Since you have quite much time, you can also enjoy the west side of Bretagne (Finistere) with a rich historic heritage too (for example 2 days around Cornouaille : Quimper / Pont Aven, Concarneau).

When we were in that area, we had a car. That makes its own problems, for example St. Malo and Dinard (which can be easily seen from each other) are on separate peninsulas with some distance, traffic, and even bridge openings, delaying travel between the two. Although you name some major medieval towns, Dinard is not one, and I'd check whether the seasonal shops and restaurants are still open in September before going there.

Without a car, there will be some attractions that you can't walk to. You didn't mention Dol de Bretagne, which is not a must-see, but we went there to visit a single standing Menhir, because we didn't have time to go to fields of standing stones farther south. But the Menhir is "out of town". In the same way, you need to make sure the "bus" to Dinan will drop you within walking distance of the old town, and not in the newer town. I just don't know, because we used a car.

September is not a real "garden" time, but there are some nice public and estate gardens in Brittany, that can only be seen by car. That just happens to be one of our interests.

I also think London is a poor idea. If the channel islands interest you, that would make more sense, perhaps by bumpy ferry.

Edit: We stayed three pleasant nights at the Reine Hortense, on the beach in Dinard, in July 2012. It was a very quiet town, and some businesses were not open every day of the week. One reason we chose the (smaller) hotel was their guest parking. Dinard has a huge U.K. holiday-home population, which is puzzling because the weather is very like it is "back home!"

Although there was a lot of driving, this was our base for MSM and Dinan. We stayed three nights outside Bayeux, for Normandy.

Just to add some precisions about Dinard : September is not anymore the high season, but all shops and restaurants are opened. Most of them are opened a large part of the year. There are buses from Rennes, Dinan or St Malo the whole year round. That's right, it's not a medieval town (the intra muros part of Dinan is), but there are pleasant walks along the seaside and villas of the Belle Epoque. Of course, Dinard is not a traditional Breton town, but still worth seeing since you have time to experiment various aspects in Bretagne. The airport is not easy to reach without a car because it's not exactly in Dinard, but near the town of Pleurtuit.

I'm driving Normandy and Brittany in September also - pick up a car in Rouen, then Etretat, Bayeux, Mont Sant Michel, Dinan (St. Malo), Lampaul Guimiliau(?) (tiny town, I can't say or spell no matter how many times I try), Vannes (Carnac) with some intermediate stops before heading to Chinon and dropping the car in favor of a rented bicycle to visit Chateaux.

I'm guessing you are talking about a bus to get around as opposed to a bus to the airport. This is my first time in France but I've been to many other places. My general feeling is buses work well if you have only one leg in a day, trying to make intermediate stops becomes impractical because of relatively light schedules and awkward connections. If you want to wake up in Mont Sant Michel and go to bed in Dinard, a bus can work fine. If you want to start in Mont Sant Michel and stop in St. Malo and Dinan for a couple of hours each before ending up in Dinard, I think it's nearly impossible by bus. You have to be really lucky to find connections that will work.

Thank you all for your input/suggestions. I rented a car in France once before and did OK navigating. However, I can relax and enjoy the sites much better when the driving is left to someone else. I plan to do overnight excursions or day trips, to take in the neighboring towns at a leisurely pace. So from what y'all have affirmed, I think the bus system will work fine. I'm not going to be struggling to make connections and such.
And despite being 'this close' to Great Britain, I'll contain myself and save London for another time.