Is London a day trip from Paris?

Technically possible on the Eurostar (I have not done it, but I know people who made the opposite trip), but not particularly recommended.

It can be a day trip from Paris. I spent a delightful 20 hours in London and have no regrets. Here are the secrets to a quick London day trip: 1) Go with LOW EXPECTATIONS. Don't expect to see much. 2) Plan everything in advance thoroughly. Research exactly what you want to do. Pick 2-3 big activities and stick with them. 3)Take the Eurostar. Buy tickets as early as possible.

Plan to go early and have a plan of what you want to see/do. On Sundays the Tube runs less frequently, so you may want to choose another day. Choose a day when the venues you want are open; there's nothing like showing up on the day your favorite museum is closed. I think a day-pass is available for Tube/bus travel; I'd recommend using one.

Rick's Paris guidebook has an entire chapter on how to do the London day trip efficiently. As long as you take the first train to London and the last train back to Paris, it's doable. Travel time is less than 3 hours each way.

As the others have said, it's doable. I believe any time in London is better than no time at all.

One suggestion is the "hop-on, hop-off" bus. It goes by all the major sights, you get commentary, and you can get on and off all day. It will give you a taste of all the main sights and you can always go back for more.

Remember, the UK is an hour behind Paris timewise so you almost get an extra hour.

Try to make your reservations on the Eurostar ahead of time as that will save you money.

I have friends that did it and really enjoyed it. It gave them a nice overview of London.

For decent fares on Eurostar, book at least one month in advance. You can purchase tickets up to six months out. Travel mid-week outside of peak times - that means crack-of-dawn early morning trains on Tuesdays, Wednesdays, and Thursdays.