It wouldn't be unprecedented for a bespoke design to be produced with the Portsmouth-Cardiff route in mind, as this is what the Western Region of British Railways did in the 1960s with the introduction of the Swindon-built Inter City DMUs - class 123 I think. They featured proper main-line standards of accommodation, including comfortable compartment coaches and a buffet car within the formation. I think they were introduced in the early 1960s, but they still looked exceptionally smart and were still hammering through my then local station at Netley when I walked home from Junior School in 1966/67 although they were replaced by MK1s and Hymek Class 35s shortly after.
It is a problematic route to serve as yes, it does act as a local commuter service throughout parts of the journey, but equally a surprisingly high number of passengers do travel longer distances, as others have said, Portsmouth to Westbury, Southampton to Cardiff, Fareham to Bath etc. A lot of military personnel use it between Portsmouth and Westbury - Navy people travelling from Portsmouth to Plymouth, and lots and lots of students between the various universities along the route. The Portsmouth/Southampton conurbation is the eighth largest in the UK, and considering the route connects this enormously-populated area of over 1.5 million people with Bristol and the Welsh Capital - the service today is positively woeful.
I have given-up using rail completely within the United Kingdom as the two routes I previously used Southampton - Bristol and Southampton - Birmingham are just so unpleasant using crowded, ancient, filthy, noisy, smoke-belching trains. I still use the Vestbanen in Norway on a regular basis and it provides such a stark contrast to the appalling non-service people are forced to accept here.