What is Britain's role in the United Nations?

The United Kingdom is a member state of the European Union..

Great Britain is part of the United Kingdom and therefore not a part of the EU,

so in simple terms, Britain is not involved in the EU, the United Kingdom is...

BUT

To answer the question specifically, the UK is the worlds 5th largest exporter with a permanent seat at the UN Security Council and is the lead country in NATO,

The UK joined the EU under the terms that it would be able to retain it's currency (the British Pound Sterling) and retain it's unelected Head of State (E II R)

The United Kingdom is a valuable asset to the European Union both militaristically and politically and is debatably the most powerful EU member both historically and politically.

However, recent incursions by the Kingdom of Spain into British Territorial Waters have caused high tensions between Spain and the UK over the British Territory of Gibraltar, Spain being a member state of the EU.

The United Kingdom will hold a referendum on whether or not it should leave the EU in 2017; and I can almost guarantee you now, the UK will leave the EU.