The official independent stats from the government, provided by the Office for Budget Responsibility, say that the UK is at least 4% worse off (in terms of GDP) than it otherwise would have been. Maybe 8% worse off.
While there have been a few economic indicators in the UK that performed okay against the rest of the EU at one time or another, the EU is made up of 27 member states, some of which are still coming up to Western European levels, and of course many of those indicators only really show that the very wealthy and big corporations have done well, not the average Brit. Inequality has been increasing in the UK.
Inequality is the biggest problem in the UK. Most of the problems blamed on immigrants are really down to inequality. Not just an unfair distribution of the tax burden, but things like foreign companies buying and asset stripping essential services. Themes Water is the poster child for that. Bought, loaded up with debt, billions and billions taken out of it in dividends and bonuses, and now it's pumping large amounts of shit into our rivers and demanding hefty bill increases to avoid bankruptcy. The government can't just let them fail because people need water to live.
It's not even new, it's just the latest in a long list of ways that we sold everything of value to foreign companies. North Sea oil should have been our sovereign wealth fund, that would have paid off the pension crisis and funded our healthcare, but instead most of the profit went into private hands.
Brexit only ensured that the EU's dislike of monopolies and plundering the economy didn't restrain them.