Here you can check valid domain name. All registrars accredited by ICANN can check validation of your domain name. You can see WhoIs-information of the regisrar (this and check who is the administrator of domain. You canl get the following information:
1. address of citizens or the location of the organization
2. the name of the provider and the address of DNS-servers
3. Contacts of administrator/owner of domain.
Dot tk domain names can be purchased from the same registrar. They have Premium Multimedia Domains: .FM, .AM, & .TV; gTLD Domains: .COM, .NET, .ORG, .BIZ, .INFO, .MOBI, .ASIA & .NAME; Boutique Domains: .LA, .VC, .BZ, .US, .MN, .IM, .MU, .CC, .NU .SC & .WS (WebSite);
123 domain names uk can be found in country specific domains:
.US, .EU, .CN, .JP, .SE, .AT, .BE, .UK, .DE, .IN, .PL, .TW, .SG, .AG, .TC, .MS, .GS, .VG, .AC, .SH & .IO.
Dot tk domain names:
Totally there 90+ Extensions, so if you search for dot tk domain names, you can register it here.