Well, this past November they did.
In December 2006, London “announced”:http://www.fas.org/nuke/guide/uk/doctrine/sdr06/index.html that it would reduce its “operationally available stockpile” of nuclear weapons (the ones on their SLBMs) from “less than 200” to “less than 160.”
It appears that the UK did this in less than a year. This past 15 November, Secretary of State for Defence Des Browne, responding to a question from Parliament, “stated”:http://www.publications.parliament.uk/pa/cm200708/cmhansrd/cm071115/text/71115w0007.htm#07111542000024 that
bq. I can confirm that we have now reduced the number of operationally available warheads from fewer than 200 to fewer than 160.
For a good resource on the UK and things nuclear, take a look at the Acronym Institute’s “Proliferation in Parliament.”:http://www.acronym.org.uk/parliament/index.htm