LONDON — For those blithely inclined toward the view that Britain would somehow find a way to sever its relationship with the European Union free of drama or financial consequences — like canceling a car rental reservation, with a tad more paperwork — Friday was a sobering day of reckoning.

As the British pound plunged some 6 percent against the American dollar in the span of two minutes in early trading in Asia, the markets offered a reminder that divorce tends to be messy, expensive and laced with uncertainties. It rarely ends happily.

The selling was so frenzied and swift that those who swap currencies for a living spoke of computerized transactions going haywire, rogue algorithms at work or a data entry error. The drop in the value of the pound appeared excessive, and it soon recovered some losses, though the British currency was down about 17 percent — around 25 cents — since June 23, the day Britain voted to abandon Europe.

More than anything, though, the precipitous drop seemed to attest to an increasingly unmistakable reality: Britain’s vote to exit the European Union — Brexit, in common parlance — has put its commercial relationships with the world on uncertain and potentially perilous ground. That poses risks for the British economy, making its money less attractive to hold.