Phileas:
So, just as an example, Barra in Scotland has instrument approaches etc. etc. etc. to support commercial air transport?
Yes it does, just as Tiree, Islay and Campbeltown do - but you won't find them in the AIP because they're approved for Loganair use only. And all of those have RFF Cat 3 or 4.
You couldn't operate a service from Cardiff to Caernarfon or Valley VFR because much of the time you'd have to route round the coast via Fishguard and dodging the missiles in Cardigan Bay and the Hawks out of Valley. The only commercial routes that do operate VFR are short routes across the sea like Orkney and Shetland inter-island.
NS