Technically it ought to be .GB , however it is one of the oldest out there so in 1985 the longer term use of domain names was not entirely known.
For .uk you can have .co.uk , .ltd.uk , .me.uk , .org.uk etc
It is interesting to read up on the history of .uk , .gb and nominet :
http://en.wikipedia.org/wiki/Nominet_UK http://en.wikipedia.org/wiki/.uk http://en.wikipedia.org/wiki/.gb
Cheers,
Rob.