@cicerone imposter,
It's foolish enough for America, but isolationism gets worse the smaller a country gets. Beyond David Ricardo's argument for free trade based on comparative advantage, there is also Adam Smith's case for free trade. Basically, the larger the market, the larger the company can grown, and the more the industry can gain from division of labor and economies of scale.
Beyond all that, America could feed itself if it had to, whereas Britain needs to import food.