If you are using Project 2000, go to the help and type multilingual features in the search box. It will walk you through the process of installing/using the multiligual features including the UK date/time. Here is a snippet.
Enable editing of multiple languages in Microsoft Project 2000
-----------------
On the Windows Start menu, point to Programs, point to Microsoft Office Tools, and then click Microsoft Office Language Settings.
On the Enabled Languages tab, select the check boxes next to the languages you want.