It seems like there is a good chance Eurostar will introduce direct trains from Amsterdam to London next year. https://twitter.com/EurostarJustinp/status/1176870778144534528
Long overdue train route. More English revelers is exactly what Amsterdam needs on weekends.
Eurostar already have a direct service between London and Amsterdam. You don't have to wait until next year.
There is a service TO Amsterdam but returning from there currently requires a change of trains and border control at Brussels.
What will be coming is a through train in both directions.
And it seems they will start in December already: https://www.independent.co.uk/travel/news-and-advice/eurostar-amsterdam-netherlands-train-ticket-direct-brexit-no-deal-netherlands-a9121816.html