London to Amsterdam - Plane or Train

My vote is for the train. You are crossing the Schengen border so you need to go through immigration whichever way you choose. Unless you're swimming across the channel, but then you risk being detained as an illegal immigrant so I would not recommend it.

The only things that speeds up the process are an EU-passport or an expensive ticket.

Global Entry (GE) is only valid when you return to the US at airports that have GE kiosks. It has nothing to do with flying between Amsterdam and London. I would take the train because you’ll be going from city center to city center thereby avoiding the hassle of airports. Plus, it will be lightyears more comfortable.

Pearzee, if you have (?) not been to Europe before, you may wish to read our host's free Travel Tips, blue menu top left in Desktop view of this website. No one anywhere in Europe or the UK cares in the least about USA Global Entry. You did not say if you are flying home from Amsterdam?

I have read that Eurostar has started (perhaps less frequent than to Paris or Brussels) premium service from St. Pancras to Amsterdam. Their may be cheaper options, but none more comfortable. Naturally, it is sold like air tickets - high walkup prices for business travelers, non-changeable, non-refundable, one-train only tickets for budget vacationers. I have used the other Eurostars a lot, but never to Amsterdam.

The 90 minute pre-check-in at St. Pancras is largely because of Brexit. But have you taken an international air trip lately? This is no worse than a check-in at a major airport these days. Amsterdam's airport has had some very bad days in the last six months. While there are bad days everywhere, there is no difference in ... quantity? ... of Immigration and Customs and Agriculture between flying and taking the train out of the U.K.

I have read that Eurostar has started (perhaps less frequent than to
Paris or Brussels) premium service from St. Pancras to Amsterdam.

I'm not sure what you mean by premium service, but Eurostar has been running trains from London to Amsterdam since 2018. Currently 4 daily departures, set to increase to 5 daily soon.

Well, one definition of "premium service" is "costs more." I don't even know if there are "boat trains" anymore, and I wouldn't use one if it existed. But you could take conventional trains from Lille or Paris onwards to Amsterdam if you had little money. And Eurostar, although it has multiple levels of service, prices, and food inclusion, is certainly a premium, even over the most express conventional trains. The product was conceived, and is marketed, like "Business Class".

Maybe another definition of premium service could include Seat Reservations Required, Eurail Passes Not Honored, or exclusively 1+2 seating. Though Emirates "domestic business" class is 2-5-2. Not so nice if you're in the middle.

I would not call Eurostar a premium service. At least not more premium than other high speed trains around Europe. Tickets can be pretty affordable if bought in advance, you can travel with a Eurail pass. And mandatory seat reservations are common on many high speed trains. And seating is 2+2 in Standard.

Another vote for the train. While I haven’t traveled your exact route, I have flown Amsterdam to London City Airport and have also taken the Eurostar (London to Paris). As others have pointed out, you will go through immigration and security whichever way you travel. For me, the train is a more comfortable travel experience in general. I agree with @Continental, the real advantage is leaving from the city of London and arriving in the city of Amsterdam. Cutting out the extra travel to and from airports on both ends streamlines your travel and can save stress. Book as early as 6 months out for the best fare. Prices just go up the closer you get to your travel date.

@Tim - the closest remaining thing to a Boat Train to Amsterdam is the Harwich to Hook of Holland Ferry. The train from London Liverpool Street takes you to quayside at Harwich Parkeston Quay. You sail overnight arriving at the Hook of Holland at 7.45 next morning. That is through ticketed including the ferry. And the ferries are very nice and comfortable.
Unfortunately NS have withdrawn the through ticketing on their side. But the tram to Rotterdam leaves from straight outside the Terminal to Rotterdam where you join a train to Amsterdam.
The through ticket is called the Dutchflyer.
Once upon a time several trains connected with that ferry including a Moscow train.
It saves you a night's hotel cost and is quite well priced.
It may not be the easiest route but is a very valid option.

@isn31c, you beat me to it ... we're taking the night ferry in mid-April ... you leave Liverpool St Station around 7 pm (which gives you a full day in London to sightsee a little more), train to Harwich, board the ferry starting at 9pm and it leaves at 11pm ... you'll get to Amsterdam Centraal about 11am ... cost for 2 is £219 (that's for a 2-person comfort class room, you can give or take a few £s depending on the room you choose ... rooms are required on the overnight trip) ... dinner and breakfast is available on the ferry but you can always bring your own ... I'm planning to schedule my trains so I have some time in Manningtree for a quick bite (a pint and bacon roll) at the Manningtree Station Buffet.