Paris to London via Normandy by rental car??

Brittany Ferries offer a number of routes from Normandy to England

http://www.brittany-ferries.co.uk/ferry-routes/ferries-france/portsmouth-caen

Assuming you will even be allowed to take a French hire car across the Channel one-way car hire will be very expensive as someone will need to return the car to France.

Best arrange to hire a new car once in England.

The only way to do that type of car rental for a reasonable price is to choose car "leasing" instead, for three weeks or more, through French Renault, Peugeot, or Citroen programs. But it's not a good plan to take a car across the Channel for just a couple of days, and you'll usually be better off to rent a new car on the other side. There are ferries from Calais to Dover and Cherbourg to Portsmouth (probably closer to the rest of your sightseeing); see www.seaview.co.uk, but again not a strong recommend from most people. Between the big cities, the London-Paris Eurostar train is most convenient. There's an awful lot to see in France.

"cost prohibitive" - probably. There is always a steep surcharge for renting a car in one country and returning it in another. This is because they cannot (legally) rent out a foreign registered car. So they have to get an employee to drive it back to it's home country, for which you will be charged.

"difficult" - not really, thousands of British holidaymakers do it every week. EXCEPT for one half of the trip you will be driving with the steering wheel on the wrong side. If it is a French car you will be driving in England on the left with the steering wheel on the left. This makes overtaking difficult.

"ferries that go from the Normandy Coast to England" - YES. The various routes are listed on this website: http://www.directferries.com/normandy.htm

Newhaven to Dieppe
Poole to Cherbourg
Portsmouth to Caen
Portsmouth to Cherbourg
Portsmouth to Le Havre

Note, none of these arrive at Dover. From the above places you have a drive of a few hours to get to Dover. Ferries to Dover go from Calais, which is not in Normandy and is a few hours drive away.

I don't know about the 'illegal to rent' part of this. We once rented a car in Amsterdam to drop in Paris expecting to pay a hefty fee to do so but given our itinerary it made sense. We were not charged the surcharge because it was a Paris car that needed to go home and so they had us drive it back for them.

Thank you all. I had completely forgotten about using a left-hand-drive car in the UK. Will need to re-think the plan.