UK vs GB

[PPCnSEO]

When selecting which search engines to monitor for a campaign both Google and Yahoo offer 2 very similar options.

"Great Britain" and "United Kingdom"

Bing only offers "United Kingdom"

From a technical point of view, what is the difference between these options and which is the better to use?

[RikkiD22]

I have always used UK but have often wondered why the two options are on here too.

The difference between UK and GB 'in real life' is that the UK includes North Ireland while GB does not, I would assume this would be the same here?

[PPCnSEO]

When I googled the differences Northern Ireland was the answer that came up.

But.... AFAIK there is no option to search google just for Great Britain.

So does google have an option to avoid NI results and if not - what does SEOMoz do differently (if anything) between the 2?