It's simple: you can recover from an economic recession, but you cannot recover your lost culture. The EU tends to be extremely pro-immigration and pro-asylum and is taking in millions of refugees and the like. I have nothing wrong with immigration, but it is imperative for it to be limited. To quote the Dalai Lama, "Germany cannot become an Arab country." I believe the exact same thing about the entirety of Europe. By leaving the EU, Britain has bought some time to remain Britain.
The issue is that a large portion of the refugees aren't adopting their host culture and, as it is in the Middle East, have a significantly higher fertility rate than most of Europe. If the refugees do not get sent back soon, Middle Easterners will soon outnumber Europeans in Europe. I appreciate diversity, but I do not want the history of my people, my race, to die out, just as a black man wouldn't want his history to die out, or an Asian man would want to preserve his culture.
Not to mention that some of the refugees are committing atrocious crimes, such as the various acts of sexual assault. For example, the one in Rotherham. Obviously I'm not some lunatic who believes that Muslims are all just criminals or rapists, but the Middle East has a vastly different view on many of these acts.