The UK has the worlds fifth largest economy by GDP and the second largest economy in the EU (Germany is larger, France is smaller) [1]. To claim that they're 'not that important globally' is, frankly, asinine. Do you really think that the EU won't also get hurt by the UK leaving? If you think it won't then it's worth asking why EUR/USD has also gone sharply down today...
[1] http://money.cnn.com/news/economy/world_economies_gdp/index....