Iceland is by far the most isolated country in all of Europe, and is hundreds of miles from any other land, making it tricky for any potential invaders to get to.
The country is also awash with useful resources for staying alive. It's powered almost entirely by geothermal energy from the country's many active volcanoes, and its coastal waters have some of the best, and most abundant seafood anywhere in the world. So in the event of having to hole out on the island for a long time, you can rely on being warm, and well fed.
Iceland also survived a near total collapse of its banking system during the financial crisis, so you know its citizens are pretty resilient, essential if the end of the world does come.