Seems to be quite a bit older than UK use, and originally US, in origin:
OED:
P1. U.S. colloq. (a person's) name is pants and variants: indicating that someone is discredited or unpopular, or has failed. Now rare.
1886 College Courier (Monmouth, Illinois) Jan. 15/2 O! dignity, thy name is pants when thou essayist to hold a candle to the Coup.
1893 Puck (N.Y.) 12 July 324/2 When things don't come a man's way right off he gits to thinkin' his name is pants.
1921 Hamburg (Iowa) Reporter 9 June, We will never be able to play another ball game and our name will be ‘pants’ from this day on for ever more.
1931 Moberly (Missouri) Monitor-Index14 Oct. 7/5 Farmer's prayer... O Mighty Hoover, who are in Washington, when not fishing on the Rapidan. Thy name is pants.