I'm in Amsterdam for 9 days. What's the best way to get to London for an overnight?

Answer 1

Flying.
The flights takes about one hour, but expect to loose half a day both directions, as you need time to travel to the airport, time to check in, security and walking through the airport and boarding 30 minutes before the flight.
At the other end you face a long walk again, passport control that can be busy, you might need to wait for your luggage and you need to travel to town, at least half an hour, can be an hour and a half in London if you are stuck in traffic.

Train, the Eurostar can be a little slower, as you need to take a train to Brussels, 3 hours, check-in 30 minutes minimum, train to London one hour and a half, and you are in the center of town, but maybe not where you want to be.

Going by car and ferry or train and ferry you will be slower still.

Both ways are likely to take about half a day, not worth it if you are in Amsterdam for just 9 days, I think, specially if it is just one overnight. London deserves at least a weekend, a week is still not long enough.
It is like being in New York and going to Washington for just one overnight.

Answer 2

Fly-about 2 hours