It looks like a big bug (very annoying). Eurostar trains require a seat reservation. Without one you won't be able to access the train at all.

Those trains also have passholder quotas and can sell out well in advance. Have a look at availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish

Reservations costs 30€ to Brussels or 35€ to Rotterdam/Amsterdam. Definitely book ASAP if there are still available seats.

I'd suggest taking a Eurostar to Brussels if available and then the hourly reservation-free IC to Rotterdam.

EDIT : All trains are full that weekend and the convenient ferry is too. I'm looking at alternatives and will update.

The Channel is a big bottleneck and it's one of the busiest weekends of the year...

Could you leave on 27th July ? This is the easiest option.

OPTION 1 (the easiest and less time consuming) :

- London Liverpool Street - Manningtree 06:00 - 06:56

- Manningtree - Harwich International 07:01 - 07:18

- Stena Line ferry Harwich - Hoek van Holland 09:00 - 17:15 (1h time difference) about 60€ per person including 30% discount with Eurail (code : EURAIL2023)

- subway Hoek van Holland - Rotterdam (not included but cheap)

OPTION 2 :

Updating...

Does it mean I need to go by ferry?

This would be the easiest option yes. Not possible on Friday as they are sold out. There are 2 ferries per day :

- daytime departure at 09:00

- overnight departure at 23:00

The overnight one looks sold out on Thursday but it's not really clearly written…

Oh no. Even I bought 5 Eurail passes which cost over 1800 pounds but can't take the train. 

Well you'll be able to take trains on other days. But definitely book in advance when required, it is peak season after all. And you do get a 30% discount for the ferry.

Which pass did you buy for it to be so expensive ?

Could you be a bit flexible with dates ? Where are you heading after Rotterdam ?

I'm still looking for other alternatives but there's not a lot of them frankly…

EDIT : There would be a Flixbus to Lille on Friday : 08:00 - 16:15 for 58€. From there you can reach Rotterdam by train.

This is the best itinerary I could find :

- Flixbus London Victoria Coach Station - Gent 08:00 - 17:15* 50€

- 2 min walk to the train station

- IC Gent-Dampoort - Antwerpen-Centraal 17:35 - 18:23

- IC Antwerpen-Centraal - Rotterdam Centraal 18:44 - 19:50

If Flixbus is delayed, there is the same reservation-free connection 1 or 2h later.

*it's a long journey but keep in mind the 1h time difference + you can stretch your legs and eat something on the ferry.