I've used a US credit card on the voyages-sncf.com site more times than I can remember. Its really not that hard. You just need to leave your pick up country as France. If you want to purchase in english go to http://www.tgv-europe.com and select the UK. Just be sure to select fares that can be printed as I am not 100% sure that you can pick up the tickets in France.

Alternately you can buy on voyages-sncf.com in english by doing the following:

1. Go to voyages-sncf.com

2. click "Recherche avancee" located about half way down the page on the left in the light blue area.

3. look at the URL in the address bar and find "LANG=FR"

4. change the FR to UK and hit return

The page should now be in english.