Hi,
You can just go to
www.eurostar.com to book your ticket.the early u book,the cheaper you have,i live in holland,when i go to uk someties.i always book it online .
in hongkong the travel agency change more than 15% of the ticket.
but i want to tell you ,if u are planning a euro trip.i would like to say you should travel with easyjet,it flight anywhere in europe,and if ur plan is very good,you book it let say 3 months- 6 months ago,the price it very cheap.cheaper then train,and very very good airline,very comfortable, and quick.
if you have more question you can email me .