There is no culture like en-UK, the culture for Great britain is recogonized as en-GB. This is a common standard across the world around i18n/i10n communities.
Luckily this package is available in Kentico. However you will have to use "en-GB" instead of "en-UK"
To answer Richard's view, US and UK English are different. I would say 95% same however still different.