There’s a Eurostar train to either Paris or Brussels every hour, so you can usually get a seat sometime on the day you want to travel without having to book very far in advance, particularly if you travel during the middle of the week.  However, Eurostar is priced like an airline with various specials, lower pricing for less popular times and lower pricing for less flexible tickets.  The cheap seats sell out first.   Check out the fares and restrictions for various types of tickets and fares at www.eurostar.com

If your railpass covers France, there is a special “passholder fare” that can be a really good deal if you don’t qualify for a youth ticket and want to travel at a time when the low cost leisure fares aren’t available.  You can buy the special passholder fare tickets at Waterloo International, the Eurostar station in London.  You can buy other eurostar tickets on line, by phone or at any London train station.

Thanks for the website and advice – I am only 21 so its going to be cheapest for me to do the youth saver i think. As i am travelling on a weekday, hopefully it will remain cheap! I will book this probably the day before.
 
Cheers[]

I’d keep checking the availability and prices for the days you’re likely to travel.  In summer, the cheap seats will almost surely be gone if you wait until the day before.