Lindsay Lohan -- No Time for Brother Cody's High School Graduation ... She's in London Town Now

Lindsay Lohan is going full Brit -- committing so fully to life across the pond that she blew off her little brother Cody's high school graduation ... TMZ has learned.

17-year-old Cody celebrated graduation with Dina, Ali, Michael Jr., and his grandma over the weekend in Long Island -- but 2 people were noticeably missing ... LiLo and good ole daddy dearest, Michael Sr.

While her bro was doing the cap and gown thing -- Linds was exactly where she's been for weeks now ... chilling in London. She's been posting selfies all over the town, and spotted clubbing and shopping.

Michael Lohan's excuse for ditching on Cody? He tells TMZ, his infant son Landon was hospitalized with a high fever -- and since baby mama Kate Major's in jail right now ... he couldn't make it to NY.

Disappointment ... a Lohan family tradition.