Is it possible to configure Safari's search box to use google.co.uk rather than google.com?
the query string is defined in : 

~/Library/Safari/Configurations.plist.signed


but the file is signed and cannot be edited without being replaced by safari.

Also, it seems that every time you use this search bar, Safari hits This file on apple.com which can make one assume that Safari compares the 2 files and replaces the local one when needed.

I think your only options are via extensions or another browser