Hi all. Thanks for all the help and suggestions, they're all appreciated.
I contacted the company that provides the SNCF Escapades card, RailEurope by e-mail. They've been very helpful, plus I now have it all in writing, just in case they try to deny what they've told me!
It seems that the SNCF Escapades card is not restricted to travel on any particular days (yay)
Secondly, while the website suggests that the card cannot be used online to book tickets and you'd have to go to the travel centre in London, or book by phone (incurring a fee of £8), you can purchase the ticket at the local station in France. They weren't sure about whether the latter would charge a booking fee, but if they do I can't imagine it would be so steep as £8.
This makes me feel a little better about travel options now, especially since booking my trips home 3-4 months in advance is not something I've ever done