Basically when you create a nameserver in a different extension and it is listed at the gtld global registry you cannot just modify a uk domain to use the new nameserver. You can in every other extension because they query the global database to check the nameserver exists and retrieve the IP automatically. At nominet you have to actually enter the IP for the nameserver along with the first use of the nameserver otherwise it never knows it exists which meant logging in and using the web domain manager rather than issuing a simple modify epp command. It could be done with a simple 'dig' command from the registry.
