Bad.
1) Britain's economy, no longer integrated into the EU, will probably take a huge hit from loss of financial sector perks which allowed London to become a financial superpower. This hurts income and thus taxes and thus govt services and also employment opps.
2) It is undoing the unification of Europe by removing a cornerstone country.
3) It weakens Britain by again casting Britain as alone, as it was between the fall of France and 7 Dec 1941...
4) Most importantly it is due to a surge in right-wing nationalism...which we supposedly defeated in WWII. This is dangerous, ppl, and we know what it looks like.