Which 2 cities to add to a London + Paris trip?

Maybe skip Versailles and add a day to the Loire. You can get your palace on just fine in the Loire, but it takes a bit of time to visit. IMO the Loire isn't the right place for a one-day trip.

One full day in Amsterdam though, totally worth it. Pick out a sight you really want to see, and then eat drink party walk your legs off in the city core. Good times. Of course you could do a lot more than a day and Amsterdam, but it is a city that works fine for a quick visit.

Maybe you could do London Paris Strasbourg Amsterdam? High speed train service is pretty good between all of them, fairly efficient itinerary without much backtracking.

Or London Paris Strasbourg for two or three nights and then fly out of nearby Frankfurt. That would probably save some time doing an air transit day in the middle of an already short and jam-packed trip.

1. Normandy (Honfleur, Bayeux) and Brittany ( Dinard, St-Malo, Dinan)

Choose Normandy over the Loire Valley.

Whether you choose Normandy (WWII Landings) or Loire (Chateaus) I seriously recommend a guided day tour in either. Otherwise, you'll waste a lot of time getting around on your own. I would add an extra day in Amsterdam. I would suggest a train instead of flying Paris to Amsterdam.

Maybe you could do London Paris Strasbourg Amsterdam? High speed train service is pretty good between all of them, fairly efficient itinerary without much backtracking.

I like this a lot. Strasbourg is wonderful. Day trip ro Colmar for wine tasting

So is your Eurostar train from London to Paris already reserved? What if you popped up to Edinburgh, then flew to Paris from there?

So is your Eurostar train from London to Paris already reserved? What if you popped up to Edinburgh, then flew to Paris from there?

That's a great idea. Edinburgh is well-suited to a one-full-day visit IMO, and I would take Edinburgh over Strasbourg any day!
Or, York is doable as a day trip from London, though not ideal.

If you really just have a day, I would favor Normandy over Loire.
I prefer the Loire, but there is more to visit in the Loire valley, so one day there would be too short.
Whereas you can easily see the highlights of the D-Day beaches in a day and leave without feeling too frustrated!
(Edited for clarity)

Balso, did you mean you prefer Normandy or the Loire Valley?

Anyway for my two cents, I prefer the Loire.

I edited, I prefer the Loire but a one-day visit to the Loire valley isn't worth it IMO. For Normandy, one day can work if that's all you have.

Stick with London and Paris.

Amsterdam is not a city you can do in one day and a lot of travel would be involved. Of course, IF you fly into Amsterdam, you would do a day, but why not fly into Paris or London?
The Loire Valley is not to be done on in a day also, with the limited time you have, save Normandy for later.

Oh good point, I forgot it was only one day.

The Loire valley has the big and famous chateau but pretty requires a car. Normandy would be best with a car too - the WWII sites are spread miles apart.

I would suggest you can could do more productive day trips from Paris such as :
- high speed train to Rennes which is a nice town, and then take a bus to Mont St Michel.
- high speed train 2 hours to Strasbourg - and another 30 minutes to Colmar. That whole area would be good to spend some days - unique Alsatian cuisine and heavy germanic influence. From there you could wander into Germany and then circle back to Amsterdam without passing through Paris again
- high speed train 2.5 hours to Avignon - a great city to hang around with good food, history and sites.
- high speed train 2 hours to Bordeaux to see the wine museum and the Nazi u-boat pens, do a winery tour, or visit the lovely stone village of St Emily.