Summer travel to Normandy from London UK

I'm not sure of the train logistics from London, but I would caution you to not get the car in Bayeux. It's just too small and limited for car rentals. So consider renting in Caen (closest to Bayeux), Rouen, Lille, or wherever your train route takes you. Otherwise Bayeux is absolutely splendid and charming!

Cheryl, have you been to France before? Unfortunately these two cities pair very poorly. I would consider a different Itinerary, but it's your trip, after all. Note that Eurostar train often requires 90 minutes for pre-boarding immigration paperwork.

https://community.ricksteves.com/travel-forum/france/england-to-france

https://community.ricksteves.com/travel-forum/france/summer-travel-to-normandy-from-london-uk

https://community.ricksteves.com/travel-forum/general-europe/landing-in-london-then-eurostar-to-paris

https://community.ricksteves.com/travel-forum/france/london-to-bayeux-and-back-again-by-rail

https://community.ricksteves.com/travel-forum/england/travel-canterbury-to-france-by-sea

Yes we have been before.
Took the Eurostar to Calais rented a car there but the Eurostar does not seem to stop there now

Have you considered flying? There's a direct flight from Gatwick to Rennes on EasyJet. You can easily rent a car at the Rennes airport and be in Bayeux in about 2 1/2 hours. Or stop off at Mont St. Michel along the way.

There was also a Summer seasonal flight this year from Southampton to Caen. There are two trains an hour direct from London Waterloo to Southampton Airport Parkway- which is as about as close to the terminal building as it is physically possible to be.
That flight was on Chalair, and was weekly on a Saturday.

There is a minor museum at the airport-https://www.ddaywingsmuseum.com/ and an Enterprise Car Rental desk there apparently.

The above list of links refers several times to sea routes it would be pointless to take from Dover. The two places are not poorly connected as claimed.
The obvious and easy route is National Express bus from London Victoria to Portsmouth Continental Ferry Port, then the overnight ferry on Brittany ferries to Ouistreham for Caen. Shuttle bus to Caen railway station where there are several car rental offices.

Yes Eurostar has not called at Calais Frethun since Covid.

I've done this.

Eurostar London to Paris Gare du Nord.

Taxi to Paris St. Lazare (train station)

Train from St. Lazare to Bayeux.

People will tell you take the Metro between the train stations. But with three people it won't cost much more and it's door to door.

We did exactly what Frank said and agree was super easy. There were several cabs waiting at the Bayeau train station should you need one.

Thanks for the report, Frank. Could you please tell us the "three-seat", rough total elapsed time, including check-in for Eurostar? Thanks.

Thank you this is helpful any more ides would be welcome
Is there a train from Paris to Caen?

Yes the train to Caen is the same as the one to Bayeux, but 15 minutes further on.

It should be 90 minute check in to Eurostar, 2 hr 30 min journey to Paris Nord, allow 1 hour to cross Paris (metro, bus or taxi) to St Lazare, then 2 hr 15 min to Bayeux- so total journey time 7 hours 30 minutes or thereabouts.
Paris St Lazare to Caen/Bayeux runs every 2 hours with some extras.

In September we did the route suggested by isn31c, ie we made our way to Portsmouth to catch the ferry over to Ouistreham (the port of Caen).
The Brittany ferry crossing was delightful - we booked one of their cabins (comfortably big enough to accommodate 4 people) which gave us someplace to retire to for cleaning up, taking a nap, generally chilling out in private, etc. As was mentioned, renting a car at the Caen railway station was easy and straightforward, and the half hour drive from there over to Bayeux was no big deal.
We originally considered the more circuitous route via the train to Paris but since we were in no particular hurry (and generally prefer slow-paced travel anyway) we opted for the ferry. FYI, we spent an extra night in Portsmouth in order to visit the D-Day museum there which was quite good.
Our crossing was the 8 AM departure from Portsmouth which arrived in France about 3:30 PM. The suggestion to consider booking the overnight crossing is a good one I think. I checked their timetable and it appears the the night crossing takes an extra couple of hours which would lend itself to a good night's sleep (in a cabin) before arriving fresh in the morning.
Ours was an altogether pleasant experience. Here's the link to Brittany Ferries if you're interested.

Another vote for the trains. But a correction (or misunderstanding?) Caen is 10–15minutes closer to Paris than Bayeux and has many more rental car options.

If you decide to take the train the entire way....a few pointers:

When you get off at Gare du Nord and head to the taxi rank you will be greeted by people asking if you need a taxi. Ignore them and join the official line for taxis. There are signs. The line can get long but it moves fast. Traffic can also be bad in Paris. For these two things, I would suggest giving yourself an extra buffer of 20-30 minutes for the transfer to St. Lazare.

Worst comes to worst, you stop for a coffee while waiting for your train to Bayeaux.

BTW, how old are your grandkids?