A Eurail France Pass is a ONE Country pass and because of that just valid in France  London is in the UK (a different Country) to travel from London to France you can use Standard Fares of Eurostar or you exchange the One Countrypass to a Globalpass (Note even with a Globalpass you still have to pay a supplement on the Eurostar London - France of arround 35€ per Seat)

Thank you for the clarification! Can you exchange for the Globalpass if I have already activated both One Country passes?

Already activated unused passes first must be deactivated by the Customer Service afterwards you should be able to Start the Refund Process.

maybe @Leo@Felipe.iturrieta or @Nanja  can clarify it :)

1-DAY passes do not even exist-or at least not for train travel.

It is as said: cancel, maybe with a fee to pay and order new one.

Hello @amcnaughton! As @seewulf mentions, the One Country Pass cannot be used to travel between London/UK and France; you'll need to get regular tickets for that trip, or exchange your Pass for a Global one. If your Pass is not used yet, it can be deactivated by my colleagues; if you wish to purchase a Global Pass and request a refund for the unused One Country Pass, please contact my colleagues via https://eurail.zendesk.com/hc/en-001/requests/new?ticket_form_id=360000363678 . To read up on the details of how a Pass Exchange works, check https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy . Cheers!