An inconsistency is that Cardiff - Manchester is part of TfW Rail but Cardiff - Portsmouth and Cardiff - Birmingham are not (being GWR and XC instead).
Given that XC is basically Birmingham Express and TPE is basically Manchester Express including Glasgow, both of course excluding London, I think Manchester - Cardiff should be TPE.
Look at it like this.
Cardiff - Portsmouth spends most of it's time in a part of the country served predominantly by GWR. It's covered by crews based in Bristol, Westbury and Fratton, and for two of those three depots those crews also work lots of other GWR services on other routes.
Likewise, Cardiff - Birmingham is worked by crews based at Bristol and Birmingham, who also (with the exception of the small pool of Bristol based senior conductors) work other XC services.
Cardiff - Manchester is worked by Cardiff, Shrewsbury and Crewe crews, who as the route is nearly all in areas where TfW operate most services, are also able to work across a variety of other routes.
There is no other logical TOC who can work the service. Whoever else you choose, it's going to be out on a limb operationally, subject to much more disruption, and also extremely inefficient in comparison to what's in place now.
People frequently complain about this Welsh intrusion into England - but nobody yet has ever been able to suggest an alternative that isn't massively flawed.