As I said earlier, the shortest & quickest route from Cardiff to Scotland (Edinburgh/Glasgow) is via The Marches (Hereford & Shrewsbury). The trouble is that the west Wales to Manchester trains on this route fulfil 2 rolls - one for long distance travellers and the other for people making short hops. So, I think that the ideal situation would be for say a 5 coach Class 800 bi-mode to run from Cardiff to Edinburgh as an express and vice versa. In addition, I would also like to see express services from Swansea to Cardiff > Newport > Gloucester > Birmingham > Derby > York & Newcastle.
(Gloucester being another place largely by-passed by the main Cross Country Express services).
Should you be travelling from south Wales to Scotland, then it usually pays to split the ticket at Crewe - otherwise, it is usually cheaper to fly from Cardiff or Bristol. (Yes, the greenest form of travel is actually encouraging people to fly this route due to high fares). See >
https://www.traintickets.com
PS. From the heads of the valleys, it is probably best to go direct by road to Abergavenny for trains on The Marches route to the north rather than go down to Cardiff.