Sales of organic food in Britain rose 40 percent in the last year, driven by consumer concern about genetically modified food and mad cow disease, according to a report by market research firm Mintel. More than 70 percent of Britons have purchased organic products, reflecting the fact that the organic industry has shed its “beard and sandals” image and gone mainstream, the report says. But just 1.3 percent of British agricultural land is used for organic farming and the limited availability of organic products is leading to high prices.