It used to be a posh upper class thing - double-barrelled surnames were often used as a shorthand for a character in a show being wealthy, for example, but it was also sometimes adopted as an aspirational thing by the middle and working class. Then over the last couple of decades you've got people getting married but neither person wanting to change their surname to that of the person they've married, so it's even more common than it used to be.
That said, the talent pool for young players in the UK is increasingly less working class-dominant, so it's not too surprising we're getting more players with them than we seemed to in the past.