I've said it before on several threads, and I'll say it again here. Great Britain never merged the Pound Sterling into the Euro, so leaving the EU is simply canceling the automatic acceptance of any rules, regulations, and migrations that Brussels dictates. It will give GB a say in those matters, and a de facto veto over policies they do not like.
If another country such as France tried to pull out of the EU, they'd also have to pull out of the Euro, and that would be much messier and have much farther reaching economic consequences than GB.
The Deutsche Mark was the largest reserve currency next to the US dollar when they merged with the Euro in January of 1999. I'm sure the Germans have something in reserve in case the Euro collapses. I don't know about the other countries in the European Union. If the Euro fails they will be in for a long depression.