Arrival time for Paris to London train

This train is an annoying exception to the normal rule for international trains.
You have to be through the ticket barrier at least 30 minutes before departure, at least 45 recommended. This is because there is a quick luggage X-ray, but more importantly you go through French exit passport control and British entry passport control before you board the train. This has queues and is always a bit chaotic due to lack of space.

You also need to allow 5-10 minutes to get from the station entrance or from the métro to the Eurostar ticket barriers, which are UPSTAIRS. Look for signs.

The ticket barrier is automatic, you hold the barcode from your ticket against a reader and it opens.
There is no customs, as both France and the UK are in the EU.

Every other international train in Europe either has no border controls, or they are done on the train, so no delays or need to get to the station early.

P.S. the official info, from the Eurostar website, is here: https://www.eurostar.com/rw-en/travel-info/your-trip/check-in

I usually arrive 1 hour before departure time even though the rules say 10 to 45 minutes depending on the class of your ticket but all your questions are answered at the Eurostar website: https://www.eurostar.com/us-en/travel-info/your-trip/check-in. I have not taken a train to London from Paris in about a year but at the time that I took it, passport control and UK border checks occurred at Gare du Nord (i.e. in Paris).

Depending on what time of day you are traveling, you may want to give yourself 15 minutes or so extra to do a quick shop to pick up some food for the train. You are not restricted on liquids so you can buy food and drink to take aboard. In Paris there are several carryout food stands after you pass thru Immigration.

Also when you are leaving Paris as you get upstairs for the Immigration area, you'll need to fill out a card with your information on it. Bring a pen with you as there are never enough. If you forget, you'll have to go back to pick up one of the cards.

I always arrive an hour before - you can get magazines downstairs and coffee, if you need it.
You will be going through Passport Control, not Customs. You will have to haul your baggage - hopefully you will be carrying light - because the train steps are very steep.
Bring a pen, because you will have to fill out your Passport Control.
If you get behind a group of travellers, it will be worrisome.
It's a good idea to look at this website to see which wagon you'll be on:
www.seat61.com/France-trains.htm
And - as a consideration, do not lock your baggage to the racks.

And one great thing about taking the train.. including this one.. is you can bring food and DRINK on the train.. no liquid restrictions..

Be one hour before. In recent months it's taken a longer time to get in due to stricter immigration procedures.

We were told to treat it like we would a flight and we did and were very happy that we did. We showed up 1.5 hours early. gave us time to get a snack and get through the security. People who followed the Eurostar advise of 30 minutes were turned away.
Give yourself ample time

I have done the St. Pancreas (London) to Gare Nord (Paris) route several times as well as St. Pancreas to Brussels. Because there is always the chance of long lines and slow security - as well as having your passport stamped - it is very smart to always allow yourself one full hour before the Eurostar departs. There are enough shops/ places of interest in the terminals so you won't be bored although once you've been past the London check-in point, it is a bit isolated although there are one or two small shops for entertainment.

Oh no, we have St. Pancreas again, that well known sainted bodily organ. ☺

At St. Pancras station, the ticket barriers open 1 hour before departure. If you try to get through the ticket barrier before then, it doesn't let you and gives an error message something like "seek assistance". When you ask a staff member, they explain you aren't allowed to check in until 1 hour before. This is to stop the limited waiting area after passport control filling up. The trains to Paris are roughly hourly, and once the previous train has boarded, you are let through.

The security is quick, and comes first. It is the two passport checks which cause long queues and delays.

Be at the station Paris Gare du Nord or St Pancras, London at least one hour prior to departure. You'll find already lots of people there.