There are currently 2 direct Eurostar trains from St Pancras to Amsterdam one in the morning and one in the evening that don't require a change in Brussels and these are cheaper than the eurostar / thalys combo. Not that I have anything against the change in Brussels its very painless but a direct service is better than having to change (which you have to do anyway in the reverse direction but it looks like that won't affect you)

Eurostar are introducing a late morning direct train from the 11th June. Prices currently £35 for economy and £79 for standard premier.