The Eurostar business lounge is upstairs inside the Eurostar terminal, but only for passengers with the most expensive Business Premier tickets, not Standard Premier. There's an SNCF Grand Voyageur first class lounge but this is only for holders of SNCF's (or other Railteam members) frequent traveller loyalty card, which you probably don't have, so of no use for most visitors even if you have a first class ticket.