How to check if reservations are available before buying a pass?

Hi community, 

I am planning a trip where I need to travel from the UK to France (via Eurostar) on a specific day - 12th September 2022.

I am aware that I need to have a reservation to travel on the Eurostar with an Interrail pass. However, when I try to use the reservation tool to check if reservations, I’m not sure because it says the pass reservations have ‘mostly sold out’ - I only need 1 so as long as they are not completely sold out I’m fine!? 

I don’t want to get a pass until I know I can travel.

I have read some other threads in the forum about using the rail operators websites to check reservation there but as far as I understand Eurostar does not sell reservations without tickets. Tickets available but that doesn’t mean they would have Interrail reservations.