Trying to plan our trip to Europe this fall. We are looking to take the highspeed train from London to Paris.
1.) you dont need a pass for it 
The pass is aimed for travelers that wanna explore several places in Europe :)
2.) London - Paris is operated by Eurostar they have advancefares that are cheaper as “Standard Tickets” but the advancefares are often not refundable.
It embarresed to say but flying with No Frills Airlines (e.g. Ryanair/Easyjet) is often cheaper or you take a 9hrs Bus ride with Flixbus :/
My experience changing travel plans because of Covid was that it was easy to change dates and times online (more than a week before travel) but impossible to get hold of anyone at Eurostar on the phone. Although flying might be cheaper, you should find Eurostar much easier to board at St Pancras than getting through an airport.
I hope that you hve a good trip.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.