From a selfish domainer perspective I think separation from the UK dilutes the .UK 'brand' in a holistic way.
Scotland might only be 9% etc but long term, in the sense to those looking into the 'UK' we're now not quite as stable as we once were, more disjointed - any sort of uncertainty can't be good for markets.
I think .uk will eventually become a great brand extension, the same as .de
and the breakup if it happens (I don't think it will ) though not desirable would not be a huge problem for the extension.