Until very recently the
UK government (or at least parts of it) used
.gb.
I used to work for DERA, the Defence Evaluation and Research Agency, and agency of the MoD, and their domain was .dera.hmg.gb. The hmg is for Her Majesty's Government. DERA's domain is now .dera.gov.uk presumably to be in line with the domain hierarchy in the US.
We still have .co.uk and not .com.uk though.