I had understood that these two sites were functionally equivalent, as long as you told TGV that you were in the UK.
However, I have been trying out some dummy purchases this morning, and the TGV site, which redirects to RailEurope.uk, does not offer the discount fares available through SNCF for the same train.
These are regular fares, not the inflated fares you'd get fro RailEurope.com (if the train was listed there at all). But no PREMS or discouts for children, etc.
Is this the expected result? Right now it looks as though the SNCF site is the only comprehensive one.