Going from London to Paris

You need to look specifically at the EuroStar, the train that goes between the two cities. It is not called the ”Chunnel.” It travels between London St. Pancras and Paris Gare du Nord. Both are relatively central to their cities but depending on where you are staying and the sites you want to see, in all likelihood you’ll need public transportation at each end. You cannot see it all, but a tour is probably the most efficient way.

Thank you. I’ll check into Eurostar and also public transportation at both ends. I’ll also check tours in Paris.

I visit both cities annually (well, almost) because there is always more to see. A commute to Paris for a day will let you barely scratch the surface. You might "see" the Louvre but have no time inside to appreciate it. Ditto for other landmarks, while shortchanging London.
But if you really must, I suggest a one-way trip. Spend a few days in Paris, then fly home from there. The fare trans-Atlantic will be about the same if you use a multi-destination search function to arrange the ticket.

southam makes a lot of sense. That is excellent advice. The Eurostar can be expensive depending on when the ticket the ticket is purchased. The money saved would pay for at least one day in Paris.

I agree with Southam about flying "open jaw," into London and home from Paris, to save the time and cost of backtracking. I also agree that just one day in Paris is way too little time, though it may whet your appetite for a future trip there.

The Eurostar takes about 2.5 hours, Here's the schedule I found for a random weekday next May. The first train leaves London at 7:55 AM and gets to Paris at 11:27 (Paris is one time zone later). The last train leaves Paris at 10:01 PM and reaches London at 11:27 (you get your hour back). Since you have to be checked into the train at least half an hour before departure, that gives you about ten hours maximum for Paris. It's more compact than London, and you can walk around the center and see a lot, but you'll want to use taxis or mass transit as well. From Gare du Nord I'd suggest taking the RER "B" train a couple of stops to St-Michel, a good starting point for exploring on foot. A one-hour Seine cruise leaves from near there, Vedettes du Pont-Neuf.

Good luck putting this together. I wouldn't do it myself. But I might add some days to my overall trip to make Paris my last stop before flying home.

It's doable to do a day trip from London to Paris. The last time I did one, we took the earliest train to Paris and the next-to-last train back to London. For convenience, we used a Mobilis day pass, and made a list of what our main priorities were. No, you won't see 'everything' (whatever that means) but you will see some things and maybe you'll like it enough to come to Paris for longer later, or you may decide it's not for you.

Read this:>https://www.seat61.com/trains-and-routes/london-to-paris-by-eurostar.htm

It’s a very tiring day and would barely scratch the surface of what Paris has to offer. Unless you have unlimited energy, it would limit what you could see the following day too.

You need at least an hour at each end for passport control. Book the Eurostar tickets months in advance for the best deals and avoid Mondays and Fridays which are busier. There are numerous bank holidays in May and a coronation, so travel during these weekends will also be busier. Please don’t call it the Chunnel - many people here wouldn’t know what you are talking about.

It's time to do some research.

No one can tell you if you can walk from your hotel to St. Pancras station in London because we don't know where you are staying. It could be a five minute walk, it could be over an hour. It all depends on where you stay.

Once in Paris, walking may not be your best bet. As an example, from Gare du Nord (the Paris train station where you will arrive), it would take over an hour to walk to the Eiffel Tower. It would take 30 minutes on the Metro (Paris' subway system).

If you want to see the basics of Paris without sending too much time there, look into the Hop On Hop Off bus. It can be slow but it drives by all the major sites. You can get off and of at each stop for the price of an all day ticket.

Trying to go into the Louvre and see the "major" exhibits will take up a lot of time.

I agree that if you can, fly into London and home from Paris. Spend at least a couple of nights in Paris to give you more time in the city.

Since you have to be checked into the train at least half an hour before departure,

I am pretty sure the required time is longer now. And remember, that is checked in (i.e. through the electronic barriers that read your ticket), not standing in line to get to the check in.