I think you need to simply convey the "reality" here and not worry about the UK equivalent (which might be the Home office) - anyway there are 4 or 5 separate jurisdictions with in the UK.

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Note added at 4 hrs (2012-01-24 20:28:20 GMT)
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Internationally comprehensible term, see:

Prison authority sorry for mishap
THE prison authorities have apologised for the inconvenience they caused a family that had paid for reserved burial lots which it later found had been used. Prison ...
www.fijitimes.com/story.aspx?id=175723 - Cached
Tax treatment of people employed in the prison service ...
Prison service: living accommodation provided by the prison authority
www.hmrc.gov.uk/manuals/eimanual/EIM68310.htm - Cached
More results from hmrc.gov.uk »
Colombia needs more jails: Prison authority - Colombia news ...
With a projected 20,000 more inmates this year than last, the Colombia's prison authority is calling for the construction of more prisons.
colombiareports.com/colombia-news/news/14999-colombia... - Cached
Nigeria: Factors Affecting Prison Authority Identified
Uyo — Lack of operational vehicles, ill-equipped workshop and none granting of amnesty to long term prisoners and those awaiting trials have been listed as some ...
allafrica.com/stories/200607010204.html
Denial of Prison Visits by the Israel Prison Authority ...
The HCJ prevents access to the courts from former inmates who are residents of Israel and whom the IPS forbids to visit their incarcerated relatives: In the judgment ...
www.hamoked.org/TopicSearch.aspx?tid=sub_21 - Cached
Colombia's prison authority to be dismantled - Colombia news ...
Colombia's justice minister confirms the country's scandal-hit prison authority Inpec will be completely dismantled and replaced by a new institution managed by the ...
colombiareports.com/.../18380-colombias-prison-authority... - Cached