Travel From Netherlands to England with rented car

Be sure your car-rental agreement allows you to take the car on a ferry.

There are ferries from Hook of Holland to Harwich. You can get schedules on the website directferries.co.uk . I have no experience taking a car between the continent and Great Britain.

are you sure that you want to drive in France, Belgium and the Netherlands with the driving position on the wrong side of the car?

Is your passenger happy to do the toll booths in France and around Antwerp for you?

You will need all the original paperwork for the car and original insurance papers - if the rental company will let you take the journey.

If you are not a holder of an EU Driving Licence you will need an IDP for France.

You also need beam benders, alcohol test kit, spare bulb kit, hi viz jackets for all, carried in the car (not back with the luggage), first aid kit and some other stuff - all required by the authorities in France, Belgium and / or Netherlands, but none of which are required to be in the car in England.

Ferry from Hook of Holland to Harwich.
Details here: https://www.stenaline.co.uk/ferry-to-holland

Yes, and I agree with all of Nigel's points. Make sure it is OK with the car hire company.
Make sure you have all the equipment (which is included if you hire a car in that country, but not otherwise)
Makes sure you have all paperwork.
Be aware steering wheel will be on wrong side, and passenger pays toll booths.

And that thousands of Brits do this every month.

Another thing that many Brits do when taking a British car to Europe for the first time is fit extra temporary mirrors to be able to see the inevitable blind spot on the left. Once you have done the trip several times you learn to compensate - the first trip can be an adventure though.

For completeness sake, from the Netherlands to the UK, in addition to the above mentioned Hoek van Holland ferry, there are also P&O ferries from Port of Rotterdam to Hull and DFDS Seaways ferries from IJmuiden (near Amsterdam) to Newcastle.

Thank you so much for all your replies! I really appreciate your all your inputs. So, looks like I have to change our original plan based on your comments. I will have to change the details of my rental car from Manchester (this is where we are flying in from Canada). I'll check if we can drop off the first rental car at Portsmouth. Take the ferry to St Malo (already booked), then get another rental car from St. Malo to Amsterdam. I will start checking into the rental car in St. Malo today. Thanks again for all your help.

renting a car from St. Malo, . . . . maybe use that rental car up to Amsterdam.

Renting a car in one country and returning it in a different one usually has a large surcharge. Sorry to add a new problem.

LOL! I just checked, yes, renting a car from St. Malo to Amsterdam is going to be quite expensive. I need to work on other options.

It may be cheaper to forget the ferry (even with a penalty), and fly from Manchester to Amsterdam. Cheap flights at www.skyscanner.com (be sure to check all the regulations for European budget airlines where extra charges add up quickly.)
As well, if it not too late, consider a multi-destination itinerary trans-Atlantic, using a multi-city search function, into England for Instance and home from Amsterdam or France.

Do you have many passengers or many places to drive in the UK? There is little need for a car for the Continental destinations you listed. We don't know your full sleeping itinerary. But the car might be less essential than you currently figure it is. Ask if you want other suggestions.