https://www.b-europe.com/EN/Booking/Pass#TravelWish

You will need a Pass Cover Number as explained in this link to make a booking.

 

Eurostar has a limited quota which can sell out weeks in advance. There are alternatives : Lille and Brussels have always more seats than Amsterdam or Paris. Use the website Yorkie linked.

Let us know your date and we can suggest alternatives when needed.

Thanks both! 
what do I need the code for? To book it on Eurostar website? Cause when I went there I was redirected on interrail.

The date I am looking for is 7th June. Sorry it’s the first time I am travelling this way and I expected to be able to book reservation since seeing it available in the planning. 
 

Eurostar cannot book Interrail seat reservations. You can either use interrail.eu (availability is only shown when you have bought a pass) : 2€ booking fee per person per train or the b-europe link : 4€ booking fee per order. B-europe asks for a pass cover number during the booking process.

I’ll have a look at 7th June. No availability between London and Paris on that day until the 14:31 train (30€ fixed price). If you’d like to travel earlier, here are 2 options :

• Eurostar London - Lille-Europe 09:01 - 11:26 30€
• TGV Lille-Europe - Paris-Nord 13:03 - 14:32 10€
• OR reservation-free TER regional trains from Lille-Flandres via Amiens (arrival 15:56)

OR

• Eurostar London - Lille-Europe 11:04 - 13:26 30€
• TGV Lille-Flandres (10 min walk) - Paris-Nord 14:42 - 15:44 10€

Check TGV availability here : https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish (same fees as above)

Eurostar and Thalys are true bottlenecks due to the quota. Afterwards only high-speed trains in France, Italy and Spain require seat reservations. You can book seat reservations on those trains until the train is full (no quota).

Let us know for questions

You cannot book reservations only through the Eurostar site, you have to use either the Interrail reservation service - for which yo need to be signed in (Charges a booking fee and doesn’t tell you if seats are available until you get to the payments area, or the B-Europe site I linked to which only shows trains with availability for pass holders. The B-Europe site requires proof of pass and that requires a number that is shown on paper passes, but not on your mobile pass. Therefore you need to use the tool I linked to to generate a PCN from your pass number shown in your confirmation e-mail - it only takes a few seconds. 

I believe you can book as many seats as you require in one booking with one PCN.