One of NATO's original functions was to deter Soviet expansionism after WWII. The Soviet Union no longer exists, but Russia has recently started to flex its military muscles. So the real question is what will NATO do if there is a serious military conflict involving Russia and one of its members?
I think there are still many citizens in the UK who can recall the serious threat posed by the Soviet Union during the Cold War years. So I think the UK will remain part of NATO for the foreseeable future.