England to France? (original plan was by ferry)

P and O ferries takes foot passengers a handful of times a day from Dover- the only company to do so.

If the car is already booked from Calais and you are ending the UK section near Dover that is your most sensible choice.

Back tracking to London for Eurostar to Lille, then train back to Calais would not be sensible, nor would flying to Paris or Brussels then train back to Calais.

Sure the "white cliffs" of Dover are not as white as further along the coast, but still makes good photography.-

https://www.poferries.com/en/routes/dover-to-calais/travel-information/travelling-as-a-foot-passenger

We crossed from Calais to Dover as "foot passengers" several years ago. It was a very moving and memorable experience - don't give up your chance to do it. We also used the P and O ferry.

I wouldn't ordinarily advocate that plan to one who was new to the plan, but in your case it seems worthwhile.

There are three main ferry companies between Dover and Calais. They are DFDS and Irish, both of whom do not accept foot passengers, and P&O who do, but only on some crossings.

You'll need to do your research as to getting on and off, because the docks on the Dover side are a long way for foot passengers, and from the docks in Calais it is even further. There used to be a P&O bus that took foot passengers on and another off, after all the vehicles had left. Read the pages carefully, count on taxis both sides between the car rental and the check in huts. And as I said, only some of their crossings do that.

That way both of your car reservations would work - presuming they are non changeable.

If they are changeable there is something to be said for Eurostar from London St Pancras or Ebbsfleet (can you return a car near Ebbsfleet? - dunno) to Paris (convenient for Giverny and Versailles), then train to Caen to pick up a car for Normandy beaches and MSM. The difficult one with a car is Versailles. Or Eurostar to Lille then car from the carpark attached to the station for Normandy, MSM, Giverny, and again the hassle of Versailles.

If workers' rights and customer service interest you, you might have a look at the last few years of industrial relations at P&O since their change of ownership to Dubai ports. Others use them, I personally don't.

I'm sure what I have written is confusing. It shows my brain battling two contrary points.

Apart from the ferries, there are also a couple of taxi companies that transport people between Dover/Folkestone and Calais through the tunnel. Their main market is people travelling with pets, but I'm sure they are happy take you even without a pet.

I have no idea how much it costs though.

Ebbsfleet has had no Eurostar services since the pandemic, with no plan to restart them.
The link given to you above goes into great detail about the foot passenger instructions at Dover.
In particular note the 90 minute check in at Dover, the same as Eurostar.
At Calais port there is a regular public bus service from the port to the town centre. That is not in doubt.
40 years ago it was far easier with boat trains both into Dover Western Docks and from Calais Maritime station, at dockside. That port was also far closer to the Centre of Calais.
There is a lot of hypocrisy about P and O. Yes, I wouldn't travel on them either due to how they treated their employees . But it is no different to how every freight shipping company treats its employees.
Yet we all consume all manner of things which have been shipped round the world.
If you were starting at London then Eurostar is clearly the best option, but as you are in Dover anyway ferry is the most logical.

Www.folkestonetaxi.com is one company who taxi across the Channel, but they use Eurotunnel, not the ferry.
I wondered about flixbus but they don't pick up at Dover, and first set down is at Lille.
Various Flixbus departures use the tunnel, DFDS, P and O and Irish ferries.
But again you have to go to London to use them and then they allow a 2 hour check in at Dover.
But bus passengers at least get a free meal on the ferry!!

There are plenty of other routes that you could consider, which will get you Normandy faster, and enable you to travel as a foot passenger too. You can consider departing the UK from Portsmouth, Poole, Newhaven, or Plymouth, all of which are on the south coast. Both Brittany and Normandy are gorgeous.

My standard warning about Channel ferries from the south coast of England to France: Sailings are sometimes canceled. I've had a 40% cancellation rate myself--2 ferries out of 5, and I was traveling between May and September. Others say I've been unlucky, but I suggest you give some thought to whether your itinerary would fall apart if your planned ferry crossing to St. Malo or other French port was cancelled. Would you have to eat the cost of an unused hotel reservation in France? Would you be able to find last-minute accommodations in Egland?

My first canceled ferry (from St.-Malo) had me sleeping in a partitioned off section of a hotel hallway. The second (from Jersey to Poole) found me scrambling for a seat on a flight to London, which required a trek from Luton to Victoria Coach Station followed by a bus to Poole. I take long, slow trips so am better-positioned than most to deal with things like canceled ferries, but tha sort of thing is really annoying.

*** Update**** Thank you for all the suggestions, we ended up cancelling the ferry and took the Eurostar to Paris and then rented our car from Versailles.