If you have a 1st class pass then you can use 1st class on nearly all services that offer 1st class accommodation across Europe, subject to any compulsory reservations required.

Res are not require on domestic UK trains but are on Eurostar.

A caveat is that on a handful of operators where trains run with more than 2 classes, Eurostar being one of these, you need to check which is valid with a 1st class pass, it is usually the case that the extra class is a super-premium offering that is not valid with regular 1st class tickets or requires an extra supplement.

1st class passes are valid for Standard Premier on Eurostar, Business premier is in the same seats but with a better meal, lounge access, fast check-in and guaranteed seats up to 10 minutes before departure, this cannot be excessed to for regular tickets or passholders.