What are the best train connections for getting from Amsterdam to Ashford International UK?
Thalys to Brussels-Midi, from there transfer to the Chunnel train to Ashford. There is only one train departure per day that stops at Ashford; 2:17pm out of Amsterdam CS.
You can book tickets, and see schedules/fares at this site.
Also try Dutch railways at https://www.nsinternational.nl/en/home
They are sometimes cheaper.