Travel from Bayeux to London

Your research is correct - there is no train route more direct than back into Paris to take a reserved Eurostar train.

There are no flights from Normandy to London. Going via Paris is the best option.

Except, if they want to be adventurous there are ferries from Normandy to the English south coast (Caen to Portsmouth, Overlord route in reverse).
Ferry company website here: https://www.brittany-ferries.co.uk/ferry-routes/ferries-france/portsmouth-caen

This is a 6-7 hour crossing, overnight is an option. The ferries leave from Ouistreham, which is north of Caen. They would have to find their own transport there. Portsmouth is easier, good train service to London. And lots to see in Portsmouth.
That would be more cumbersome than going via Paris, but would be a good fit for a D-day themed trip of a lifetime.

Flybe has service from Caen to London Southend, but that's quite a ways from London itself.

I would not recommend cross-Channel ferries for novice travelers. Two of my five Channel ferries were canceled, leaving me scrambling. I know I was incredibly unlucky, but there is some risk there.

Have you/they considered a day tour of D-Day sites from Paris? They wouldn't see as much as they would on an Overlord tour from Bayeux, but it would sure be a lot less stressful for a couple of novice travelers.

If it were me, I would spend the night of the 19th in Bayeux and take an early train back to Paris on the 20th; then grab a taxi from Gare St. Lazare to Gare du Nord to catch the Eurostar.

https://loco2.com/en/journey/bayeux-london-12ap0ft?currency=USD&outbound=undefined&leg=outbound

Thank you all for the wonderful information and ideas. I looked at Chris's information on the ferries and it seems to be the most convenient and cost effective (sleeping on the ferry). I am concerned by acraven's experience with the ferries being canceled. Was this the same time of the year? What did you do to recover and get to London?
I also appreciate the loco website, I had not seen that. I will speak with my cousin in the next couple of days and give her the options. I would really like to hear how acraven solved the cancelation problem so I can give them a back plan. Maybe the back up plan can be the train Traylaparks provided. I am open to any ideas or critiques. You guys are great. Thank you so much!!!!

I would love to take the ferry from Ouistreham to Portsmouth some day. But I wouldn't use my suggestion as Plan B - because Eurostar trains tend to sell out.

Sure wish I had a cousin like you.

The first cancellation was ages ago; it was a ferry from St. Malo to Jersey in the Channel Islands. I had to find a last-minute room in St. Malo and ended up sleeping in a partitioned-off section of a hallway. At least I had a bed. And I had no room booked on Jersey, so I didn't lose any money, just a bit of time.

The second cancellation was summer 2017. It was a ferry from Jersey to Poole, England, by way of Guernsey. I found out about the cancelation the night before scheduled departure. I had a non-cancelable hotel room in Poole. It would have been smarter to simply abandon it, but instead I found a flight from Jersey to London-Luton on EasyJet (not horribly expensive, fortunately), took a bus to Victoria Station in London and another bus to Poole. Not a fun day, but despite my prior experience I hadn't created a back-up plan.

Neither of those solutions would work for your travelers, but as long as they don't have a tight schedule and decide to depend on a ferry, they should be fine. What happens if they're a day late getting to England? Do they just meet a welcome dinner?

thanks acraven. I appreciate the time you took to answer me with all that information.

Does anyone know how they would get from Bayeux to Ouistreham?

Let's get a few things straight: Normandy is poorly connected to London. Ferries are an obsolete relic of another age. Ferries can have brutally rough waves and wind. Normandy is "best" visited by car. (I love trains ... ...) Brexit could wreak havoc on anything you plan.

The original train station change in Paris you planned is the best idea for your "clients."

Now for the Opinion: I understand that many Americans think that the most important thing that ever happened in France was the Normandy invasion. But it does not pair well with London, where everybody speaks English ( of a sort.)

Thank you Scythian. Tim had me kind of freaking out there. Not sure what he meant about his comments about Normandy, but I think I will just move on from here. Thank you for the link showing the launch dates of the ships.
Does anyone know the best way to get from Bayeux to the port for the ferries????

www.skyscanner.com

You can fly from Caen to London with one stop, averaging five to seven hours. Not cheap, but neither will be the total for the two train tickets and transport between Paris train stations. Skyscanner has good coverage of short-hop routes in Europe, and links to appropriate ticket-sellers. It doesn't matter which city you change flights; what counts is the elapsed time and of course what you are paying. Always better to buy the whole itinerary on one purchase.

There are non-stop flights from Caen to London, although not every day. Totally anecdotal, but 10 years ago I booked a ferry from Poole to Cherbourg. When I got to the Poole Ferry terminal, they informed me the fishermen were on strike in Cherbourg, and hence my ferry was cancelled. The ferry company took me and my traveling companion from Poole to Portsmouth by taxi, and got me on a ferry to Ouistreham (close to Caen). My guess is that taxi cost them more than our ferry fare. They even made the arrangements to change my Hertz rental pick up from Cherbourg to Caen, and arranged for my transportation from Ouistreham to Caen. In the end it worked out, and I suppose if I had made train arrangements they could have been on strike too. But like I said, anecdotal. YMMV.

Thank you Phil,
What a great story!