Two searches are performed when you do an availability search for a domain.
The first tries to resolve the domain name to an IP address - if this fails, then the system assumes that it is available.
Then upon adding the domain to your shopping basket in our shop, a slower direct Registry search is performed where the domain is then found. This will result in either the domain being available for purchase or showing as already registered.
If .co.uk domain name goes past it's expiry date it is detagged i.e. de-registered but it still owned by the registrant under Nominet rules. To purchase such a domain it is necessary to contact both the .co.uk Registry Nominet and the Registrar.