An extension for Safari which redirects searches from google.com to google.co.uk
UK Search Safari Extension
As of version 6.0, Safari still doesn’t have a setting to automatically use google.co.uk (or uk.yahoo.com) for searches. This extension redirects searches from Google.com to Google.co.uk, and from search.yahoo.com to uk.search.yahoo.com.
It doesn’t do anything else, there are no settings and it’s free (source available on GitHub).
Update Get the latest version from Github for Safari 6.0.1.