I am not quite sure. I always thought that .co.uks were for UK based companies and not as desirable as doman names, but when I asked before in another post people told me that they had sold many co.uk domain names so I am sitting on a fence with .co.uks.
Also, the desirablility of the domain names is dependable on the domain name. For instance, what I have found is that fj3.co.uk will not be as desirable as cat.co.uk and therefore, not as valuable.