Be very wary about buying tickets that include local services and long distance trains - even where the only company involved is TfW. Check out a pay on the day return from say Fairwater in Cardiff to Llanelli on the TfW website. Then check it out on
https://www.traintickets.com/?/and you will see what I mean.
Split ticketing is a huge issue across a lot of the network. I think TFW has a lot of work to do to ensure that people don't lose out. It could be as simple as automatically applying the cheapest option with split tickets.
Take Merthyr to Llanelli leaving right now (a route I have done several times).
On TFW's website, they only offer the anytime single which costs £25.10.
However, I can totally legitimately buy:
Merthyr to Bridgend anytime single - £5
Bridgend to Gowerton off-peak day return - £7.60 (£9.30 if I travel during the peak and need to buy an anytime single)
Gowerton to Llanelli anytime single - £4
Total = £16.60
That's a saving of £8.50 - a massive difference given the fare.
It would have huge revenue issues on this flow though. It would also cause issues as the split ticket itinerary wouldn't be valid on GWR services between Bridgend and Gowerton.