LONDON—U.K. Key Minister Boris Johnson is being investigated by the country’s electoral watchdog subsequent allegations that he failed to declare who funded an upgrade of his residence at 11 Downing Avenue, the latest of a series of political battles around his conduct and that of his authorities. The Electoral Fee, which regulates political donations, reported Wednesday it had been in contact with Mr. Johnson’s Conservative Bash following concluding there “are fair grounds to suspect that an offense or offenses may have occurred,” amid allegations that undisclosed donors provide money for a refit of his official residence. Mr. Johnson reported he has personally coated the expenditures of the apartment’s face-elevate. “I compensated for it!” reported Mr. Johnson in the…