EuroStar Train ticket.. Buy it in HK or in London?

I am going to visit London in around Chinese New Year time. If I want to go to Paris from London, where should I buy the EuroStar Train ticket? Will it be cheaper if I buy it here in Hong Kong or I can do so when I am in London?

I found that the booking in StaTravel web-site is quite cheap. But do I have to be the member for making booking there?

Thanks!