1. First install google toolbar from http://www.google.com/toolbar/ff/index.html
2. Restart firefox
3. right click on Google toolbar, select “Toolbar options”.
4. Click on Search from the top icons, now on this page change “Use Google site” option to google.co.uk
5. Click on Layout from the top options, now select option “Replace firefox search box and hide Google toolbar”.
6. Click save, done.