WSIM is used to create an xml file which can be used to automate installations of Windows 2012 servers.
The default settings (the ones that you may find in official documentation) are heavily biased towards US.
Here there are the settings that will get you going if you need to use the GB locales:
Components->[windowsPE->amd64_Microsoft-Windows-International-Core-WinPE__neutral
InputLocale: 0809:00000809
SystemLocale: en-GB
UILanguage: en-US
UserLocale: en-GB
Components->[windowsPE->amd64_Microsoft-Windows-International-Core-WinPE__neutral->SetupUILanguage
UILanguage: en-GB
Components->Specialize->amd64_Microsoft-Windows-Shell-Setup__neutral
TimeZone: GMT Standard Time
