That Britain is not currently threatened with nuclear Armageddon as it was during the cold war does not mean that such a threat could never again occur. Britain must remain prepared for any eventuality which has to include the unthinkable such as the United States no longer being an ally.  The world is not yet a safe place there are many unstable states, such as North Korea, developing nuclear weapons capabalities. Beyond these dangers it is easily conceivable that the world will once again face tensions similar to those of the cold war. Given the length of time it would take to rearm should such tensions occur Britain would be safer to keep its nuclear armament.