Many BREXIT opponents point out the immediate potential economic costs, many of which come in the form of blackmail or extortion by the EU anyway (and amount to just more reason to get away). However, what gets ignored are the massive economic costs already paid by Britain by being part of the EU. There are enormous opportunities here but it is up to the government of Great Britain to take advantage of them.
Creating a competitive, innovation-friendly atmosphere is a huge potential hidden “win” of Brexit—with correspondingly huge opportunity costs from failing to do so.
Source: The Unseen Economic Benefits of Brexit - Foundation for Economic Education
The above article is now four years old but I think it still holds true.