.uk.net is not a Top Level Domain name but appears that it is just a .net which someone registers the domain name as .uk.net and offers you the sub-domain as a service. Why not just use .co.uk or .org.uk TLD instead ?
I guess the only place to get it is http://www.centralnic.com/registration?aid=H122681 because they are the registered owners of UK.NET
If .uk.net was a proper TLD it would be available on 123-reg - they have all the UK extensions available.
Checked it out in google just out of curiosity and this one https://www.hostinguk.net looks like a legitimate valid seller of uk.net (There is a domain search box on top with lots of extensions, uk.net included)