Since there’s a Eurostar train about every hour, you can usually get a ticket to travel on the day you want, however specific trains do sell out.  You can buy Eurostar tickets at any of the London train stations.  You can also buy them by phone, on-line from www.eurostar.com and from some travel agents.  Many Eurostar tickets can be exchanged for a different day and time with no penalty if you do it before the original departure time.

Potential problem:  Eurostar is priced like an airline.  There are specials, leisure fares and other “cheap seats.”  Some of these fares are only for off-peak times like the middle of the day on a weekday.  If you want to travel on Friday or the weekend (especially a holiday weekend) these won’t be available.  Of course, the cheap seats sell out first, leaving only the regular fare and first class tickets which are quite expensive — particularly when compared to the cheapest flights.  You can check fares, fare rules and availability at www.eurostar.com  (a good deal if you don’t qualify for a youth ticket and can’t get a leisure fare) you will have to buy the ticket in person at Waterloo International station.  I tried several other options, but could only get the passholder fare at the actual Eurostar station.