Trains help? fromlondon, to paris

For tickets see https://www.eurostar.com/us-en

For more information see https://www.seat61.com/

Yes, to horsewoofie's links and will add:

• Book well ahead for the cheapest prices
• Allow yourself some flex time between transportation modes...for instance if you are flying in to London, don't think you can just head to the train station and catch the next Eurostar. It might be more difficult than that.
• Consider flying open jaw - in to London/home from Paris or the opposite. No point in backtracking on this route.

Do we need to go through customs as international passengers on eurostar? Is Eurstar the best train to take?

Yes, you will go through immigration and customs since you are traveling between two countries. It will be done BEFORE you get on the train.

Eurostar is the only train you can take between London and Paris.

Because Immigration formalities AND emigration formalities occur at the point of departure, it is like a plane -- you need to get there well in advance as lines are long. 90 minutes is a reasonable advance arrival but look at current recommendations.

Eurostar is the ONLY train to take. 2.5 hour ride.
Expensive so book early to save some money.

Should be at London’s St Pancras station at least 2 hours in advance. You can dine, drink and shop at St Pancras. I’ve enjoyed the ambience of the Booking Room before taking the EuroStar to Paris.