<p>How can I change the system-default regional settings in windows XP for use by services (run by the system user)?</p>

<p>Regional and Language Options in the control panel modify the settings for the logged-in user. However, services don't use the user's settings - they use the system settings. I know that they can be found in the registry here: </p>

<pre><code>HKEY_USERS\.DEFAULT\Control Panel\International
</code></pre>

<p>My question is: What mechanism is there for changing the system-language/date/etc from en-us to en-gb?</p>

IM not sure if this will help
First type in gpedit.msc from the run command and a dialog box should now open. 
Navigate to User Configuration > Administrative Templates > Control Panel > Regional Settings.
This shows the group polcies for the PC/s, maybe you can just set it so en_gb is the only option!!
Group Policy is not my strong point, but do a google search and go from there, I would imagine that you should be able to set up who and what can do what from here.
HTH