Microsoft doesn't have an "en-GB" localization like some other operating systems do. They do appear to have UK-specific marketing and technical support pages and associated departments, but they don't have a Language Pack for British English.
The language strings are stored in a not-very-user-friendly
.mui file in the language pack. The problem is that
MUI files are covered under Windows Resource Protection (WRP), which means that whenever the files are altered, Windows will automatically detect this alteration as "corruption" and attempt to restore it to the factory default, or the latest patched version of the file from a Windows Update.
More reading about MUI here
More reading about WRP here
It is technically possible to bypass WRP but that's shooting yourself in the foot if you ask me. It's not worth bypassing a critical self-healing feature of Windows just to make the UI look "right" (to you) ;)