The abstract principle of national sovereignty has been made very real by this crisis.
The Eurosceptics are in the ascendancy. They may be mostly Tories, but they have been proven right. In previous recessions, the troubled eurozone countries -- Greece, Portugal, Ireland and Spain -- would have eased their economic problems by devaluing the currency, printing money (now called "quantitative easing") and cutting interest rates. These levers are no longer available, so they have no alternatives to immediate cuts in public spending, together with steeply rising unemployment, falls in wages and drastic reductions in benefits.
Thus, as the Eurosceptics warned, a single currency leads to loss of sovereignty, which sounds like an abstract idea until you get a crisis like this. It applies even to the more economically successful countries. If the eurozone is to survive, Germany may have to keep rates lower than it might wish, risking inflation. Eventually, like Greece, it will have to accept greater EU control over public spending levels. Neither country can run a wholly independent fiscal policy within the eurozone, any more than Cardiff or London can within the sterling zone.
You could argue that, with proper democratic controls, a single European treasury would be good for everybody. But the pro-Europeans never made that argument, preferring to insist that the loss of sovereignty was an illusion.