You can't see availability before logging in (annoyingly). It only lists prices.

Eurostar has a passholder quota and some trains (especially in the mornings) can sell out a few days/weeks in advance.

Which train(s) on which date are you trying to book ? I'll gladly help. :)

You can better check availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish (4€ fee per order compared to 2€ per person per train on eurail.com btw)

If London - Paris is full, check London - Lille-Europe. You can connect there to a TGV to the south of France without the transfer faff in Paris.

Ugh, that's so annoying! I forgot to include this in my first post, but it's not even the London to Paris that is full--it is Paris to Barcelona. I just tried to do Lille to Barcelona and because it goes through Paris, it is full. The train I was trying to book is 

TGV 9715

TGV INOUI

@thibcabe^

I'll gladly check for you. On which day ? I can think of a few alternatives.

@thibcabe October 15, I would like to leave London early in morning and arrive in Barcelona before midnight. Thanks so much :)

So something weird is going on... the 14:56 TGV from Paris still has 2nd class regular tickets on offer so why doesn't Eurail offer reservations ? There was a bug a few weeks ago on eurail.com but it looked like it had been solved…

(@Mukhammad @Camilo. could you have a look please ? Thanks!)

If you're able to do so I'd suggest calling SNCF (the French railway company). They speak English and can book a seat reservation for that train without fees. See : 

I wouldn't wait too long to book those 2 trains since it's a popular route and weekends are always busier than weekdays.

I'd leave London at 09:31 to have a good 2h buffer in Paris. Remember that you need to arrive at London St. Pancras 45 min in advance due to passport controls, this is unusual for European trains though.

Let me know how it goes. :)

Thanks so much! I will do that tomorrow. 

@thibcabe they fixed it! thanks so much for your help

Glad it worked. Thanks for the feedback :)

Do you mean eurail.com or SNCF call center ? Just so I know whichever option works best !

@thibcabe eurail.com!