(Reuters) - Britain's economy could grow by 1.3 billion pounds if it left the European Union due to less regulation and more trade with emerging economies, a British diplomat who dreamt up a prize-winning blueprint for the country's EU exit said on Tuesday. Britain's free market Institute of Economic Affairs on Tuesday awarded a 100,000-euro prize to Iain Mansfield, a British diplomat based in the Philippines, who it decided had come up with the best blueprint for a 'Brexit,' a British departure from the EU. Amid widespread public disenchantment in Britain about the EU's perceived over-bearing role in everyday life...