Ok, for a start, By Expired you mean "Suspended".
First, Check to see that it isnt a Pre-Nom (Pre- Nominet) domain name. These names are SLOWLY but surely being brought back in to circulation.
See Here for more info -->
http://www.nominet.org.uk/registrants/prenominet/
Secondly, .CO.UK domains are notiriously hard to get hold of. The competition is more fierce than that of .COM names. Unlike .COM names, .CO.UK names drop randomly... they are unpredictable. You could be waiting up to a year in some cases for .CO.UK names to finally drop. At which time they WILL (99.9% of the time) be snapped up by registrars that manages to get "tags" before nominet was around. People have set up systems, and there are LOADS, but they keep them private so that they can catch the names and keep them (And never use them, not even park them... grrr how annoying!).
If its half decent you can bet your life that the systems below wont catch the names for you:
http://www.123-reg.co.uk
http://www.dropcatcher.co.uk
http://www.snapnames.co.uk
http://www.pool.com
... All hopeless.
All of the people that are actually able to get the domains are behind Closed doors. Notorious ones being "CENTRESERVE", "BIG" "QUO-VADIS" and such like. You can even contact them and they won't reply.
It all comes down to Pleeding with the owners of these systems and paying them lots of cash to make it worth their time.
.CO.UKs are a vicious game.
- Luke