By David Milliken LONDON (Reuters) – Extra trade barriers created by Britain’s exit from the European Union and subsequent trade agreement have added 6% to the cost of food, researchers from the London School of Economics and other universities estimated on Wednesday. The research compared price changes for food imported from the European Union with prices of food from further afield. Food mostly imported from the EU, such as fresh pork, tomatoes and jam, saw bigger price rises than those such as tuna fish and pineapples which mostly come from elsewhere. “This research demonstrates a clear and…