Let's say the UK leaves the EU.
What are the practical, short-term and medium-term effects? How does life in the EU and life in the UK actually change?
(The UK never had the Euro, so that doesn't change...)
Hard to answer, it really depend on how the aftermatch is handled.
A Norwegian/Swiss solution: The situation will stay like now, except that London's value as financial centre have becomed much lower. We will likely see a lot of international companies moving their European headquarters to other countries. But in general it will have little effect except for slightly lower growth. Of course this solution demand that UK keep uphold the free movement of labour. The political consequence of this deal will likely be chaotic.
A clean break/free trade agreement: It will be pretty damn horrible.