I live in Belgium, and it has been my experience that it's almost impossible to get affordable Eurostar or Thalys tickets at the last minute for anything that even resembles a reasonable fare. Just a quick check on the website search engine shows fares anywhere from 450 to 650 euros any Sunday between now and the end of June. ONE train from paris to London has 80 euro tickets available for 2 July, and that's it. Sunday is a popular travel day on trains.

I would *strongly* advise you to purchase your Eurostar tickets as soon as you have your travel dates set. It sounds like you'll have everything else arranged fairly well in advance. Enjoy your trip planning!

Carrie in Belgium