Count me in with the other replies who have taken the train from Paris to Rennes to pick up a reserved rental car. We really enjoyed the drive out to Mont St Michel and all around the coast. We stayed on the Mont, so the car stays in a large paid parking lot on the mainland, and they have free shuttle buses to take you from the parking lot to the Mont.
After Mont-Saint Michel, I next need to get to England. Will it be tricky to get to Saint-Malo? Or would it be better to aim for another place - which city would make the most sense to travel to next to board a ferry?
We did that Channel crossing after finding out about the ferries. We booked on Brittany Ferries, who depart from several ports in France to several ports in England. Pick a port based on where you want to land in England. We did not leave from St Malo because we wanted to drive the car around and explore more first, but if you need to go straight to England, Brittany Ferries sails from St Malo to Portsmouth or Plymouth. The ferry ride was fun, but the Channel can sometimes be a bit of a ride if you have problems with seasickness.
St Malo is close to Mont St Michel so if you had a rental car it would be an easy short drive. However...you do have to account for which city you will be able to drop off the rental car. That affected our route planning. The final piece in the puzzle is transportation from the rental car drop-off location to the ferry dock.
Book everything well in advance...we traveled off-season but still ran into crowds on Mont St Michel and less choice than we expected for rental cars. August would be nuts!
If you can schedule this and if weather allows, try to be able to explore Mont St Michel in the evening and early morning. During these hours the crushing mobs of day-trippers are not jamming the very narrow streets, so the whole place is much emptier and more like what we might imagine it to be. We only stayed one night but managed to enjoy it this way.
An alternative not discussed above is train to Paris, change to the gare du Nord and take Eurostar to London. From there you can take trains to almost everywhere in England, Scotland and Wales.
We considered that, but one reason we went with the ferry is that the ferry crossing would only be 5 hours, while in comparison, doubling back to Paris, crossing the channel, and getting from London to our eventual England destination would not only take many more hours, but cost much more money in total fares.