Okay, so I finished COS, and the only refrence to anything American I found was on page 19, Chapter 2 (Dobby's Warning): "From the dining room he [Harry] heard Uncle Vernon saying, '...tell Petunia that very funny story about those American plumbers, Mr. Mason. She's been dying to hear...'"
It seems that Gilderoy Lockhart is indeed British.
XDD
American
Plumbers
I want to hear the story!