Jippie, Johnny
The high speed train from Amsterdam will certainly not compete with LCY-AMS route. There will not be a direct train from Amsterdam to London, one will have to change trains in Brussels. Journey time from Brussels to London by train is about 2 hours, plus another 2 from Amsterdam to Brussels. Flight time from AMS to LCY is less than one hour, so that makes it a total no-brainer.
LH