Free trade with a bunch of obligations and restrictions attached to it isn't free trade. It's trade with a bunch of obligations and restrictions.
Free trade simply means eliminating tariffs, trade quotas, and the like. A country can simply eliminate its own tariffs, and that presents a major step forward. That country can enjoy cheaper access to the tools of production, and thus become more productive.
As a simple example, Brazil and Argentina could eliminate their self destructive taxes on computers, and immediately allow their productivity to increase as more people have access to modern computers.
If you encourage other countries to also eliminate their tariffs, that helps free trade even more. But beware! Once you get into negotiations, you often end up adding a bunch of government wastefulness and burdensome obligations. Big corporations and big unions will demand special favors in exchange for supporting lower tariffs. Usually, those special favors end up costing almost exactly as much as the savings from lower tariffs. As an example, look at TPP.
The EU isn't free trade. With the EU, access to lower tariffs comes with major obligations and restrictions. There are the direct costs of the EU fees. There are the nearly guaranteed costs of bailouts (given that at least some of Europe's socialist economies are collapsing.) There are the welfare costs of having to support basically anyone who comes to your country.
There are the lost free trade opportunities with other countries. As part of the EU, Britain could not set up free trade agreements with other countries (e.g. the US). Free trade agreements had to be with the entire EU, or with no one at all.
Any divorce can be a little messy at first. But if you were married to 27 deadbeats, who felt that they had the right to barely work, retire at 40, and spend all of your money on their laziness...dropping that dead weight would help you out. And even if one of those 27 deadbeats (Germany) actually worked, you could still divorce the group and start a new relationship with any one of them individually.
Even better: you can now set up new relationships. You've learned not to share a bank account with people who lie about their finances and debts (Greece). You've seen that other people can get the same benefits without the burdens (Switzerland). You realized that the most successful people (U.S., China, Japan) weren't a part of your 28 person expensive marriage.
Britain is now free to pursue better opportunities. It no longer has the massive obligations of carrying around EU socialist deadbeats.
It may still fail. After all, Britain has its own wasteful welfare socialism to eliminate. But it will now succeed based on whether it embraces free market capitalism or welfare socialism, not based on whether other nations embrace free market capitalism or welfare socialism.
Libertarian National Committee