Are you asking for the best combination of walk-on fares? (that is how it has been interpreted so far) If so, it depends on what time of day you want to travel, are you combing back the same day, etc?
Or are you asking for the cheapest Advance fares?
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Yes, for Cardiff-Birmingham you can split at Cheltenham or Gloucester (it makes no difference).
There is no Off Peak Day fare from Cardiff to Cheltenham so if travelling back same day you can get the Anytime Day from there, and then Off Peak Day from Cheltenham or Gloucester. This would be valid on the 08:45 from CDF.
A through ticket from CDF to BHM on the 08:45 would cost a shocking £62.00, compared to £33.50 by splitting. That's a saving of nearly 50%!
For Cardiff-London,
as documented before in this section splitting at Swindon and Didcot is cheapest (for more details do a search for previous topics, or download the
Avantix Traveller software and have a play with that to see the fares). Going via Salisbury is slow, but have a play with Avantix and see if there's an option worth doing - as suggested by Techniquest splitting at Salisbury might work.