Both sound rather good; but I just believe that Carlton just didn't do it because Central was a well known name in the network, while Westcountry was a newcomer.
If TSW had kept their franchise, I could totally see them using the Carlton 1996 identity.
That doesn't make sense really. Surely being a well-known name would've saved Central over Westcountry.
The reality is that the idents for both Carlton & Central were made back in 1996 before Carlton gained control of Westcountry. For whatever reason, the Central versions weren't implemented, presumably due to internal politics with Central's management. Clearly, by April 1998, this obstruction had been removed and so the idents were belatedly introduced.
None existed for Westcountry and so they were left alone. Also, by 1998, it had already been agreed that there would be a switch to dual identity ITV network branding and so there was no point designing anything new for Westcountry in the meantime.