As much as I understand your frustration, let me try a suggestion. On my computer, the Google page with a search box (google.com) is a start/home page. What if you make a start page the Google UK page (www.google.co.uk)? This way, when you open Safari at the home page, it will be a UK page directly and you can search immediately the way you want. Alternatively, you can put the Google UK page on the favorites bar and open this page with one click from the bar. This way, no plugins are required.
Previously the solution was to install Safari plugins Inquisitor or Glims but Apple have removed the interfaces from Safari 4 that these plugins used - and there is still no built in Safari Preference to set the locale of the Google search.
Effectively this renders the Safari Google Search field pretty well useless - at least for UK users. If you're searching for UK-based businesses or individuals, for example, the thousands of USA results that come up first are just not relevant - (that's an example of British Understatement. by the way