london to normandy- travel on sunday?

I can't answer most of your questions. But about renting a car in London to drive in France, I'd be nervous too, because the steering wheel of a UK car will be on the wrong side for driving in France. It's legal and Brits who do it regularly have tended to say, here, "oh, no problem." However, my personal experience (driving an American car in Asian countries that drive on the left, like Thailand) is that this is a bit nerve-wracking--because the driver is sitting on the wrong side of the road to see around the car ahead, when trying to overtake and pass on a 2-lane road. It makes passing a car interesting, and virtually impossible if you don't have a passenger in the front seat to see around the car in front of you, because you (the driver) can't see if there is an on-coming car until you get the car all the way out in the opposing lane.

It costs extra to return a car in another country, assuming they allow you to take it there (i.e. Left-Hand drive ... ) Best idea is to change your departure day. I agree that a car is useful in Normandy. However, Rouen is not the high point of that area. It is a good place to return a French car and walk around for a day. Have you considered training to, say, Bayeux, where there is a day of things to do without a car? Or Lille, which is a Eurostar stop? People here say Caen is a better place to rent a car, not Bayeux, however. How about taking a passenger ferry to St. Malo or Cherbourg, again good for a day on foot? Can you do Paris first? I'm sure it's easy to get rid of a car in a ferry city, or Calais. Are you also going to Mont. St. Michel?

One more example of how renting a car in Europe is different from the convenient option that it is in N. America:
In this case, the traveler is blind-sided by car rental locations being closed on Sundays, which would not happen here. Not intended to be a criticism of Europe, just that renting a car there is different and/or sometimes more complicated than here.

And trains are not necessarily simpler, at least in the planning stage, as many of us are not familiar with how to:
1) read the on-line schedules
2) whether to pay in advance or on the day of

thank you for these helpful suggestions. i have found i can go to Lille and rent a car, assuming the Europecar information is accurate. i am wondering why the drive through Normandy is difficult? Are roads not well-marked? Point of having a car was to do some independent exploring before being in paris for 4 days.

I, too, have used a rental car to explore Normandy and the D-Day Beaches.
I don't remember the driving in Normandy to be especially difficult. I use a GPS and detailed map while driving anywhere in Europe.
You'll want to avoid driving in the larger Normandy towns (there aren't too many of them), if possible.
Usually the most challenging part of renting a car is the first half hour or so where you pick it up in town and are trying to get out of town and on the open road.
French country roads use a distinctly different system of signage than here: Instead of saying (for example) I-10 North, in France especially in the country, the sign does not give you the road number and compass direction, instead it lists the names of the next couple of villages in a particular direction. This can be confusing at first, if you're not expecting it. No big problem if you have a GPS--but even with a GPS, it's helpful for your navigator to tell you the next couple of villages in the direction you want to go, before you get to the intersection or traffic circle (I'm referring to country intersection or traffic circles). Some drivers even write down the next couple of village names on a post-it note and stick it on the dashboard.