Eurostar to Paris

All formalities are BEFORE boarding, and you ignore the 90-minute rule at your peril. You will not be allowed to board if you arrive close to departure time. Same on the return trip, if you do that. They are not kidding. Search this newsboard if (blue box top center) if you are hoping for a different answer.

Don’t ignore what the EuroStar website states!

Customs is not involved. It’s security and immigration!!

So arrival 2 hours early at St Pancras would be sufficient then? Can we count on a one hour train journey to be within 30 minutes of the stated arrival?

all one can say is that barring unforeseen delays your train will arrive in London on time

You'll also need to find your way from the platform you arrive to the gathering point for Eurostar departures. I have no idea how much time you should allow for that.

Customs as well as emigration and immigration are all handled at the point of departure.

I'm not sure if you are arriving from Sussex to St Pancras on South Eastern High Speed or on Thameslink.

But the latest statistics from South Eastern are that just 0.9% of trains (1 in every 100) have been cancelled in the last year, and of the trains that ran only 0.05% (1 in every 200) were over 30 minutes late.
The figures for Thameslink are only marginally worse.
Statistically you are fine. Of course you can never rule out a longer delay but you have to be reasonable, you can't plan for every possible contingency, only every reasonable contingency.
So the compound chance of a train cancellation followed by a train being over 30 minutes late is incredibly small, almost minute
Eurostar are currently saying 90 minutes hard deadline for standard class, 45 to 60 for Business Premier.
Ultimately it is your assessment of risk.
Personally I would give myself an extra half hour to reduce the probability to well under 1%, but that is absolutely me.

Customs is not involved.

Customs is very much involved as the trains between London and Paris cross the EU customs border.

I haven't taken this train, but I booked a first class ticket that I got refunded. I seem to remember that first-class has a much shorter arrival window. Something like 15-minutes? Maybe they have a priority line or something, but like I said, I never used my ticket. I bought first-class because I knew there was a good chance my plans would change (they did) and I needed a fully refundable option. YMMV.

So we will have our passport stamped at St Pancras as opposed to upon arrival at Gare Du Nord? That is what customs means to me and that would be very different then to how air travel between countries works!

There's customs and there's immigration/passport control. Two separate things.

Passport Control/Immigration is for you. It's where your passport is stamped. And yes, it will be at St. Pancras.

Customs is for your stuff.

When you arrive at St. Pancras, you will go to the Eurostar departure area. You will go through security, then French/Schengen immigration. Your passport will have been stamped.

When you arrive in Paris, you just walk off the train and go.

I haven't taken this train, but I booked a first class ticket that I got refunded. I seem to remember that first-class has a much shorter arrival window. Something like 15-minutes?

@Pharmerphil, things changed a lot since 2021 and the implementation of post-Brexit regulations. Now you need 45-60 minutes even with expensive business premier tickets, and 90 with other tickets.

I’ve recently taken Eurostar between Paris and London, using expensive business class tickets. (Someone else booked and paid). I didn’t need 45 minutes. And my ticket information told me as long as I got there within about 20 minutes I’d make it. However, you do want to allow time to go visit the lounge that you pay for as part of that business ticket so I would give yourself at least 45 minutes just to make sure you have time to visit the lounge.

What Balso says is what Eurostar themselves say, they also add that the departure gate is closed 15 minutes before departure for business- 30 for standard. It is like an airline. You can choose to follow the carrier's guidance, or run your luck. The fates may be on your side, but you have little or no comeback if you have not followed the carrier's instructions.
For clarity this is the Eurostar page- https://www.eurostar.com/uk-en/travel-info/your-trip/check-in

Neither of your 2 recent posts say >where< in Sussex. East and West Sussex add to a lot of space.

If you are on Southeastern's Javelin high speed service into St Pancras International you will arrive upstairs, slightly north of Eurostar check in. Down the lift or escalators, around some restaurants into the main central corridor, the next perpendicular corridor to the left is your destination.

If you are on Thameslink (Brighton, Eastbourne, Lewes, Gatwick etc., (change for some off Southern) it comes into the sub basement of St Pancras. Up a (somewhat slow) lift or pair of escalators to the main corridor across from where folks coming down from the Javelin services, and then follow their directions. It is all very well signed too, but there is a lot of signage so may take a minute to get your bearings.

If you are out in the bergs of rural Sussex you will probably be on a slow Southern train into Victoria, then Victoria Line to Kings Cross St Pancras.

If you share where you are coming in from we can help more.

Do NOT arrive late at Eurostar. Pleading will get you nowhere...

We are staying in Henfield - would be taking the train from Burgess Hill in to St Pancras. I believe the rail line was Thameslink…

I traveled that route on April 4th - we arrived around 10:15 for a 1:30 departure (we really thought it would take longer to get there by Tube with all our luggage but it was super fast) and couldn't check in until 11:30. So lots of waiting in the terminal. Once we were allowed into the line, we went through passport control (took about 10-15 minutes total) and then waiting in the departure lounge. We had to stand until the 12:30 train started boarding and then were able to find some seats; our train started boarding around 1pm or so. Arriving at St. Pancras between 7 and 7:30am should be plenty of time.

Once you arrive in Paris, you just step off the train and go.

Sarah, don't make assumptions about current immigration and customs based on either past experiences or romantic novels. The last time I landed in London, my passport was never stamped. I used an entirely electronic gate that took my picture and then opened the gates to let me walk into the U.K.

I forget that particular entry, but an awful lot of customs checks in Europe consist of choosing a red door or a green door. Alas, people who look unusual or very ethnic tend to get selected for an actual interview.

I am afraid that your question about 2 hours suggests that you don't have a lot of experience with public transportation. Nobody can promise you that your local train, or your international train, with meet its schedule. Even in Germany, we had a train with a passed-out drunk on the floor that delayed our departure for the airport so long that we got off and took a much more expensive taxi. And I once had my Eurostar (London-Brussels-Antwerp) delayed eight hours because the previous train had struck and killed a pedestrian in France. Luckily, Belgium held the last train to Antwerp for the Eurostar, and I got back at 1AM or something like that.

I'm not saying that is going to happen to you. But you are looking for a Victorian-era certainty about train travel that does not exist today. Just like in the U.S., politicians and business managers are reluctant to spend the money. Why? Because every time the public is offered a choice between "service" and "price" (or "taxes"!), they choose ... ... ...

The way to protect yourself totally is to buy a train ticket from Burgess Hill to 'London Intl (CIV)' [which is destination code LNE], not to London St Pancras. I don't think you can do that on line- it has to be done at a station.
Under the Convention Internationale pour le transport des Voyageurs (CIV) rules if you miss your train and have allowed the 90 minute connection Eurostar have to convey you forward on the next available service. Be sure to retain your ticket when you pass through the gates at St Pancras (Thameslink).
On a Monday to a Saturday I would take the 0625 from Burgess Hill due 0730, on a Sunday the 0556, due 0731.

By the way I have just followed up on this CIV ticket- up here in Cumbria the destination shows on the machines as London Eurostar (CIV), Station Code LOE.
I have even proved that the code works by buying a dummy fare from Cumbria to LOE, and have also got it to show me Burgess Hill to LOE (it admits the fare type exists) although our machines don't have the actual fare in.
Quite why the code differs from that shown in the Fare Manual I am not sure (it's not the first time I have encountered such a variance), and it is still not available on line, but I have physically validated that the fare works.
EDIT- Having had more time to follow this through this morning the fare is £3 more than a normal London fare, advance fares (where applicable) are still available, as are railcard discounts, so the add on is only £2 with a railcard. A small price to pay to guarantee your connection. And the Avanti machines at Carlisle have the fare in- it is £24.80 on the peak trains as a walk up fare, so you can easily buy this guarantee fare.

As Tim notes, it seems unlikely you'll get an actual stamp anymore. Like him when I arrived at Heathrow I followed the ceiling signs with the little American flag, went up to the ATM and laid down my passport, they took a picture and the gate opened.

There was a sign on the wall to the effect that they want to keep track of everyone entering the country but don't need to actually see or talk to you. I'm guessing France is not too dissimilar.

You can also watch Youtube videos from vloggers who have taken that route, just make sure they are fairly recent. IIRC it can be quite the maddening crowd at St. Pancras.

“I'm guessing France is not too dissimilar.”

France still stamps in/out. I traveled Eurostar yesterday from Paris to London. I went through an egate for France, then a France Immigration officer stamped me out, then an egate to enter UK.

My arrival was to Amsterdam where they did stamp my passport to enter.

Heck, I arrived at Heathrow and had to go through the staffed passport control (traveling with a minor) and we didn't get a stamp on our passports either. We had a stamp everywhere we traveled, in/out Iceland, in/out France, but not into the UK.

Good to know about France and the in/out stamps. I can count on getting that. Hopefully in Germany too, if they are obliging.