Both are used in British English.
In my experience (don't shoot me!), I'd say that there's a nuance in meaning:
"I'm on the football team."
means I'm part of the squad, regularly go to practice, and play in a lot of the games
"I'm in the football team."
implies that I've been selected in the starting 11 for the next game
That's how people where I'm from use it, anyway.
We're one, but we're not the same.