For eight years, TTT Moneycorp Ltd. regularly took Dariusz Suchicki to dinner and soccer matches, all while selling him, as the head of finance of a U.K.-based importer of Polish foods, a series of complex currency derivatives.

When the pound started moving sharply against the zloty, those instruments became toxic: They began costing Suchicki's company, Best Foods, tens of thousands of pounds a month, the company said in court filings last year. Now, as Britain prepares for Brexit, blow-ups of this type of complex financial instrument are surging. 

About a quarter of all U.K. small and midsize enterprises routinely use hedging tools to manage currency risk, broker Afex Markets Plc found in a survey carried out in 2015. Many of these instruments are highly leveraged and can result in magnified losses for companies hit by big currency swings.