After purchasing a Eurostar ticket to go from London to Brussels and my flight is delayed so that I have to catch a later train. Would my ticket be still valide? Is my ticket only valid for a specific time of travel or is it flexible on the day?"
It depends on the type of ticket you bought. E-mail them through the Eurostar site below if you can't see an answer to your question.
http://www.eurostar.com/UK/uk/leisure/customer_care/email_us.jsp