I may have mentioned this before, but Bristol Temple Meads - Weymouth (in summer at least) and the Cornish Mainline on the regional DMUs would surely be a hit too.
Other potentials would be Exeter to Barnstaple (A lot of end to end travel with journey time of just over an hour, also seems to have the 'right type' of passenger likely to purchase something) and possibly even the Newquay branch in summer, as I believe there are a lot of long distance passengers using that service (the locals probably use the bus) and the facilities at Par are lacking.
I can't really think of any more on GWR. Major flows on Cardiff to Taunton are Cardiff - Bristol - Weston - Taunton and don't really require catering. I also probably wouldn't bother on the Golden Valley locals.
The catering offering is substantial on the TfW (ATW) network, I think catering is more or less on every service except the locals/West Wales, Machynlleth-Pwllheli and The Valleys.