Remember that although a .uk domain may be detagged, it still technically belongs to the Registrant and is not available for general sale.
Nominet will eventually contact the Registrant to confirm they no longer want the domain and if they don't, the domain will be made available for sale after a varying amount of time.
R.
However, Nominet are now speeding up the process considerably and also numerous names are going straight to suspended status without ever getting detagged. We have recently seen some such names, including valuable single word generics, be deleted 6 weeks after having been in full use and with their search engine positions still intact.
It basically means that although the domain is still registered to the respective company, it is not hosted by anyone and will not work.
A domain which is detagged is most likely this way because the owner has not renewed it, although this isn't necessarily the only reason!
After a varying period of time, Nominet will contact the domain's owner to see if they wish to renew the domain. If they don't, it will be made available for sale after another varying period of time.
Hope that all adds up - it's been a long day! :(
R.