Eurostar:  Use www.eurostar.com to book in advance.  Look for cheap leisure fares (more likely in the middle of the day) on the day you want to travel. If you end up buying a railpass that covers France, you may find that “passholder fare” is the best deal for the date you want to travel (likely if you’re over age 25, not the case if you qualify for a youth fare).  You’ll have to buy that, in person, at the Waterloo international train station since that’s not sold on-line or over the phone — although you could buy it with your railpass.

You need to look at the individual national rail company sites to price these tickets: www.sncf.com for France, www.trenitalia.com for Italy, www.renfe.com for Spain.  These sites don’t sell most international tickets, so you may have trouble pricing those a ticket for a route that crosses a border.

You could also try www.railsaver.com   Either use your likely itinerary and get a recommendation on which, if any, rail pass is best or use smaller portions of your itinerary (so it doesn’t recommend a railpass) and get an estimate of the ticket costs.  In either case, follow the “I have a good idea of my itinerary” route and click “I prefer purchasing rail passes (to point to point tickets) ‘only if it saves money.’” If you don’t see the “only if it saves money” option click on “more destinations” (even if you haven’t filled in all the destination blanks) and it will appear.