The way international relations work nowadays there's no default of free trade. You need specific deals in place for trade to occur. Britain is, like pretty much every country, very dependent on trade and now that it isn't getting trade deals as part of the EU it will have to suddenly negotiate its own.

Also, a lot of recent British prosperity comes from London being the banking center of Europe. Countries tend to handle most of their banking internally but the exceptional connectedness of the European system let Britain take over more of the market then it would otherwise be possible to do, across national boundaries.

But on the other hand, stock markets in France and Germany have gone down further than those in Britain. And stock markets in Southern Europe even more so. So it looks like a lot of the market is really concerned about what this means for the recent Eurozone trouble and whether there'll be more countries leaving. Or more likely there are reasons that aren't clear to me.

But as a famous Brit once said there's a lot of ruin in a nation and this won't be the end of the world for Britain. I can imagine someone looking back on this in 30 years and saying it was all for the best, possibly. But 5 years out I don't think anyone will be saying that.