Easiest/cheapest way to book train from London to Amsterdam (Leiden)

this tells you what to do - go to

https://www.seat61.com/Netherlands.htm#by Eurostar

then click on the "London to Amsterdam by Eurostar & onwards train"

I guess you're saying that the Eurostar site only wants to sell you a ticket to Brussels. So you could possibly be on trains of two countries, Belgium and The Netherlands, that don't have reservations. Actually, I would expect the long-distance trains to have seat reservations, but I haven't tried that particular route.

Since London has three airports, have you considered a low-budget airline? I'm not saying it's better, it's just more predictable about seating and luggage space .... and luggage fees, of course! I'd worry more about luggage space on local trains that seating. One problem is that some local trains are "commuter" trains with double-decker cars and narrow aisles. The over-seat racks only fit tiny hand luggage. Amsterdam and London are not well-linked by rail. I have been to and back to London, from Antwerp, and even that isn't so convenient. I was really lucky when there was a rail-crossing fatality in France, after the 8-hour delay, they held the last train to Antwerp for the Eurostar arrival. (Not saying that happens every day.)

I would buy a Eurostar ticket to Brussels, and use the NMBS (Belgian) site, International journey dialog, to buy the next leg. Pricing is directly affected by amount of lead time and non-changeability for Eurostar. In that respect, it's like an air ticket today.

Actually going to NMBS now, I see that they (international dialog section, as I said) are willing to write a single ticket from St. Pancras to Leiden, NL. Crucially, some of the journeys are marked Thalys from Brussels to Rotterdam, which means your exposure to lousy seating is quite short. Only as far out as June 15, the price seems quite reasonable. (Without Thalys, i.e. all IC type local trains, its only 59 Euros, non changeable.

So I would bet that the Netherlands train website would also sell the entire journey. I leave that exercise to the reader. However, it would be most "strategic" to buy the Eurostar journey separately for best control of seat choice, travel bonuses, and so on. Thalys tickets are certainly available by themselves. But then you have two transactions.

Note, although it's not material to your booking I think, that Eurostar arrives at Brussels Zuid/Midi, not Brussels Centraal.

My husband really likes this site and booked all of our tickets there. They provide boarding passes for your phone, and other conveniences like that, as well as great train schedule info.

https://www.trainline.eu/

Have you actually tried booking on Eurostar.com? I've tried looking from England and they can definitely sell tickets for the full journey.

We've booked that exact train trip for later this month - all on NS International. We're stopping at Schiphol Airport as we have an early morning flight, but there's also a stop at Amsterdam Central.
https://www.nsinternational.nl/en/traintickets#/

Thanks. NL has a working call center.
Do any of you know if it's hard to get seats at the time we're going?

get seats on a Thursday in June at 4PM? I don't fancy standing for 2 + hour

Get seats where? Which segment do you mean will be at 4pm? That is 2 hours long?

I'm confused. Eurostar is all reserved seats. That's London to Brussel-Zuid. Thalys is all reserved seats. That's Brussel-Zuid to Rotterdam or Schiphol. At Rotterdam take the next train the few minutes to Leiden. Or stay on to Schiphol and backtrack one giant step on the next train to Leiden.

Which bit do you expect to start at 4pm? If it is the little run to Leiden, yes at 4 pm some trains will be busy. They are very short connections.

If you plan this far out you may still be able to get the Thalys at a reasonable price. The slower International InterCity train was not reserved seating the last time I looked but I've never seen it SRO.

The Brussels - Rotterdam segment. Thalys is way to expensive twice the price.. We taking the IC train instead, which get's in a little later.

So does anyone know how crowded this train gets? Thanks.

Perhaps a better subject line would have been "Rail tickets bought, how's crowding Brussels to Leiden?"

But I never know where to post these inter-countries questions.

I haven't looked at the Brussel-Zuid to Antwerpen Centraal loadings, but from Roosendaal to Rotterdam all the trains around that time show loadings today of one figure out of three on the Dutch NS app. That's very lightly loaded.