Your best chance for registered Golden Guernsey goats would be through the Golden Guernsey Goat society, there are contact detals on the website. Google "Golden Guernsey Goats UK".
At this time of year you may be more likely to get young kids, especially if you speak to owners before kidding.
Be prepared to wait, and in the meantime go to the nearest goat shows and talk to breeders there - still the best way to see stock and make contacts.