MacOS – Can the Safari 6 omnibar search be changed from google.com to google.co.uk

I don't use it, but GLIMS looks like it will easily let you insert a search engine different than what Safari bundles.

More intrusive, but way slick in my opinion is the SafariOmnibar it's excellent work and you can customize your search provider since it's hacked into that part of safari using SIMBL - the SIMple Bundle Loader process.

I use LaunchBar and search from a keyboard shortcut - it allows extensive control over searching - you can define custom search sites and criteria easily. It has many other features (notable extremely quick app and file launching as well as a clipboard history) but it's so fast you might like it just for getting a UK search if nothing else fits your needs.

From a few articles I found, it sounds like this isn't directly supported in Safari.

However, there is a Safari extension that will redirect google.com to google.co.uk and should take care of your problem. (This is a simple extension, using the built-in mechanism, and not a plugin.)