Having had a quick eyeball at the BR Fares website, it seems that TfW are the fare setter for the Route 01000 (is that the code for Any Permitted?), which has both Standard and First Class fares. Was this one formerly set by InterCity in the days of British Rail?
Intercity lost interest in the Marches route in 1970 when WCML-origin services to South Wales/West of England via the Severn Tunnel were merged with those from the North East via Birmingham New St - which became Cross Country.
After that it was a secondary regional route, split between 2/3 BR Regions, and has latterly served South Wales only, and not Bristol/South West.
At the lowest ebb of BR, Marches trains terminated at Crewe with connections further north, even to Manchester.
Through trains to Manchester and Liverpool gradually returned, and later were concentrated on Manchester.
On the South-North Wales route via the Marches, there were no through BR services on the route, a change at Crewe was needed.
Wales & West did introduce a single Cardiff-Crewe-Llandudno-Holyhead service in the 90s.
Fares were previously set, often to different rules, by RR North West, Central, and Wales & West.
That changed when ATW began to run all local services in Wales from 2003, though I think you can still see some of the "joins" of the previous setup.
At privatisation, from what I remember the XC (Any Permitted) fares were the only through fares to Manchester/Liverpool, valid on any route including the Marches.
Dedicated Marches fares are a relatively recent thing.
There are still "Intercity" fares on the Marches, eg Shrewsbury-Reading via Newport, set by GWR and significantly better value than the TfW fares on the route.
I've not checked, but there are also probably Avanti fares from Marches stations to northern destinations on the WCML (where the TfW mileage is less that VT's).
