If you want a description of the experience, then I can do my best, having done it many times. You check-in at London St. Pancras (or at Paris Gare du Nord on the return journey) at least 30 minutes before departure. This means that you press the barcode on your ticket against one of the ticket gates (or show it to Eurostar staff; sometimes there is a short queue at this point), then go through airport-style security scanning and then show your passport to French police. This normally only takes around 10 minutes. Then you are free to wait in the departure lounge underneath the platforms at St. Pancras where there are a few shops, you can get out money or simply wait on the usually ample seating. About 15-25 minutes before the train leaves (exact time seems to depend on when the train is ready) boarding is announced and everyone (the trains are 18 carriages long and can hold many hundreds of passengers) goes up one of two escalators to the platform (the escalator used depends on which end of the train you are booked in - Eurostar tickets are only sold with reservations). When the train leaves you travel through tunnels for about 10 minutes, then continue down to the Channel Tunnel on High Speed 2 at up to 300 km/h, crossing the spectacular Medway viaduct. Some trains stop at Ebbsfleet or Ashford. The trains enter the Channel Tunnel after around 35 minutes. The time in the tunnel is normally around 18 minutes and as far as I know the train travels at 170 km/h. After leaving the tunnel the train travels non-stop to Paris at 300 km/h along a high speed line to Lille and then the LGV Nord (some used to stop at Calais-Fréthun or Lille-Europe but these stops are now exclusively served by London - Brussels Eurostars). You will hardly see Lille as you pass through it mostly in tunnels. The northern French countryside is mostly very flat, particularly in the Nord-Pas de Calais region (Calais/Lille area) and so the journey is not particularly scenic.
When leaving Paris Nord you will need to pass through both French and UK Border Agency passport/visa (possibly for non-EU nationals) checks and go through security. In Paris you go up an escalator from the main concourse to go through these checks in an area above ground level on the western side of the station before redescending to the platforms via escalators (they are blocked off when a Eurostar is leaving).
On the London - Paris route on-train announcements/announcements in the departure lounges are bilingual English/French, with the language of the country the train is physically in coming first. All Eurostar staff speak both languages well.
You can save money on Eurostar tickets at the moment by choosing a version of Eurostar's website which allows you to buy in euros as well as by joining Eurostar Plus Points (or Frequent Traveller for business travellers).
See Seat 61 for another description (with photos).
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To get back to the original subject, I remember seeing something recently amidst the revelations about two Advance Purchase tickets being used in succession being treated as one journey that not only London Intl/Estar CIV + Eurostar but also Advance Purchase to London Terminals + Eurostar is treated as a through journey. Do this remove the advantage of CIV tickets of being put on a later train in case of delay? Or is this only really relevant to UK-bound journeys as Eurostar may not follow these rules?