Pirates used whatever currency they could get their hands on which was often not British, partly because they were often attacking and plundering non-British ships. They famously used "pieces of eight" or Spanish silver 8-reale (dollar) coin as a standard of currency and they weren't the only ones to do so. Even in the North American colonies in the 18th century, Spanish dollars were so common as a unit of currency that this is where the term "quarter" comes from because people would cut Spanish dollars into quarters, as well as eighths known as "bits" which is where the term "two bits" comes from. The first US silver dollars were in fact based on the Spanish real. The main reason Spanish dollars were extremely common and widely circulated in the Americas was because Spain produced them on a massive scale from their massively wealthy silver mines in Mexico and Peru starting in the mid-16th century.

Since all currencies were based on precious metals at this time, none of this really mattered or caused any problems. If you had an ounce of silver or gold, that was worth the same amount everywhere regardless of what country produced it and stamped their markings on it. So in answer to your question, pirates used a variety of coins from different countries and often a good portion of those would have been Spanish.