why does Google default to South Africa but I'm in the UK??

I'm on a Safari page and decide I need to look something else up in Google, so I click the magnifying glass icon top left the current url grays out and I get an oblong dialog box with a magnifying glass icon at the left and x at the right. I enter my search details and click the blue Google box bottom right of the screen. Invariably I'm through to http://www.google.co.za/....and I get region specific results first. Weird.

When I google from bookmarks or web app, I'm fine.

Any ideas