It is not necessary to hack the registry, it can be done using common settings. I actually had the same issues when I switched my language from English US to English Canada. It seemed the bloody US English took any changes to make itself as default anywhere you go. :-(
Anyway, eventually it disappeared. :-))
A checklist for you (better in the order). Make sure
Control Panel | Regional and Language Options:
1. Regional Options - Standards and Formats: English (UK)
2. Regional Options - Location: UK
3. Languages - Details - Settings - Default Input Language: English (UK) - US
4. Languages - Details - Settings - Installed Services: English (UK) ----- Make sure English (US) is deleted
5. Advanced - Language for non-Unicode Programs: English (UK)
6. Advanced - Default User Account Settings - Checked
Microsoft Office Language Settings
7. Editing Languages - Available Editing Languages: English (UK) ----- Make sure English (US) is removed
8. Editing Languages - Primary Editing Language: English (UK)
Microsoft Word / Excel / PowerPoint
9. Status Bar: English (UK)
10. Language: English (UK)
11. Language - Default: English (UK) ---- Must save the template after changing default language
11 places to go!!
Good luck,