I found this post on Eurocheapo's blog quite helpful:
I should think that usually it would be easier to use tgv.com, saying you are from the UK, to buy the same tickets.
Is there something I am missing? I can even get TER tickets on the TGV site, not that I would bother most of the time.
If you go to www.tgv.com and choose the UK, you will be bumped to Rail Europe's UK site.
If you go to www.tgv-europe.com and choose Great Britain as your country of residence, the site will remain in English, and you will not be bumped to RE. You can book the same discount fares that are on www.voyages-sncf.com.
Tim, your advice is always the best!