Implying that ~ 55% of its exports go outside the EU, and since the EU is a customs union, external trade incurs tariffs by default and members are prohibited from negotiating their own trade deals to change that. The 55% number is despite those constraints; it's not unreasonable to think that the proportion would be quite a bit higher if EU trade had tariffs and trade with the rest of the world didn't [1]. The 55% number is also rising steadily as EU growth lags behind the rest of the world. I'd be surprised if Brexit ends without free UK/EU trade, but even if it did I don't think it'd be the disaster many are painting.

Short-term things will be rocky given the uncertainty and adjustment pains; long-term things will be fine; medium-term could go either way depending on how the various parties behave.

[1] That's simplifying a bit, since the EU does have free trade deals with some countries, although notably not with the US or China or India or much of the rest of the Commonwealth. Also, some of the UK's trade with the rest of the world is currently routed through the EU, though given how cheap freight is these days that shouldn't be too hard to change if necessary.