There is for example the "Computer Misuse Act" from 1990, which criminalises "unauthorised acts with intent to impair, or with recklessness as to impairing, operation of computer etc.". Now tell me how sending a message that you know will crash an iPhone in a way that is hard to recover from isn't an "unauthorised act with intent to impair, or with recklessness as to impairing, the operation of a computer".
"Prank" and "hacking" are orthogonal concepts, so saying "it's not hacking, it's a prank" is nonsense. "Pranks" that involve the Computer Misuse Act are not something you should get yourself involved in.
The difference is that this _is_ a criminal act, whereas smoking marijuana doesn't actually turn you gay, and most people wouldn't care if it does. But the next time you come up with a strawman argument, think of something more clever.