Do you have to be at the station a certain time in advance for Eurostar services? Or can you just rock up a few minutes before and jump on, much as you might do with many other European high speed trains?
There is a minimum checkin time for Eurostar trains, which is generally 30 minutes before departure (see below for details), but it does vary.
Before listing the times, it's worth thinking a little bit about how the Eurostar checkin process works, as that explains why it is as it is. When you get to the station, you need to checkin with your ticket, either using the automated ticket gates, or with one of the checkin counters (normally only if you have a problem). The deadline is for going through here. After that, there's normally a small queue (varies with the number of services due out) before the security screening. You can take a lot more onto the Eurostar than a plane, but there are still some restrictions. Your bags get x-ray'd, you go through a metal detector, then you queue up again for the juxtaposed border controls. You show your passport, and clear immigration for the country you're going to (either France of the UK, depending on direction). After that, you're into the waiting area before boarding (and frequent travellers can visit the lounge)
Boarding of Eurostar trains normally begins 20 minutes before departure, and they tend to close the doors a minute or two before the scheduled departure time so they can leave on time. For the ski trains, and summer Avignon service to the south of France, boarding starts earlier, often 35 minutes before.
For most people, on most services, the minimum checkin time is 30 minutes before departure.
If you're travelling on a Business Premier ticket (the really expensive ones), or you have a Eurostar Carte Blanche frequent traveller card (lots of journeys in a year), then the minumum checkin is 10 minutes before. However, as you do still need to clear security and immigration, you're probably going to have to run to the train, and there won't be time to go in the lounge with only 10 minutes, so getting there earlier is probably advised, especially at busy times!
For the long distance services to Avignon and the Ski Trains, the minimum checkin is 1 hour (60 minutes) before departure. Boarding starts earlier for these, and most people have more luggage, so it all takes a bit longer hence the extra time. Anyone with special travel needs needs to get to the station 75 minutes before.
The minimum checkin enforcement varies a little bit. If it's quiet, you may get away with 25 minutes, but that's about it. At a very quiet time, you can clear security an immigration in about 5 minutes, but at busy times with lots of queuing it'll take longer. At a busy time, they tend to be pretty strict!