While hard to quantify exactly, you are largely correct that most pirates during the "Golden Age of Piracy," (late 17th-early 18th century) were indeed British. Pirate historian (yes, there is such a specialty) Marcus Rediker has compiled a database of 778 captured pirates from this era, which includes the pirates' birthplaces, when known. While useful, this database probably has a British bias, because Rediker focuses more on the British Atlantic world in his research. In Villains of All Nations: Atlantic Pirates in the Golden Age, Rediker uses this database to estimate that 47.4 percent of captured pirates during this era were English, and about 1/3 of these were from the Greater London area. Many pirates were also from British port cities like Bristol, Liverpool, Dublin, Glasgow, etc. The next largest group--about one-quarter--were American (which during this era would have included the British West Indies as well).

However, there certainly were Spanish pirates, as well as French and Dutch pirates, and a fair number of pirates of African descent. These international pirates represent about 6.9% of those recorded in Rediker's database. However, Rediker believes that this is likely an underestimate, especially given the frequent references during this era to the multinational aspect of pirate crews. It was a common trope during this era that pirates held no national loyalties at all. One British colonial official in 1697 claimed that pirates "acknowledged no countrymen, that they had sold their country and were sure to be hanged if taken" (Rediker, 53). However, it's hard to find evidence of pirates themselves actually expressing this view themselves.

As to the question of why the British were more likely to turn to piracy than other nations, that's trickier to answer. One potential explanation is that British Americans saw piracy as advantageous to their financial bottom line: pirated goods were far cheaper than goods brought into the colonies legally under the various Navigation Acts. Mark Hanna's Pirate Nests and the Rise of the British Empire, 1570-1740 shows that British Americans, in fact, exhibited a widespread acceptance of piracy in the late 17th century. For example, Hanna notes that Pennsylvania's Lieutenant Governor William Markham married his own daughter to a notable retired pirate, James Brown. Such relationships show that, for a time at least, piracy was moderately socially acceptable , even amongst colonial elites (so long as the pirate in question was wealthy). Hanna goes on to argue, however, that the British state launched a concerted effort to crack down on piracy beginning in the 1690s, including a anti-piracy propaganda and legal campaign, which eventually eradicated colonial support for the institution. By the 1710s, pirates were no longer welcomed in former "pirate nests" such as Newport and Philadelphia, turning pirates into the outlaws that the British state had long claimed them to be.