Can I cancel SMS sending on iPhone while it is still in progress?
Unfortunately, it's a done deal when the Send button is hit, and there is no native way to cancel it, or delay the sending, so the user could have the opportunity to cancel.

If after pressing send you swipe upwards from the bottom of the screen and hit the flight mode button (top left) then the message will not be able to send. You need to be reasonably quick, but if you practice it a few times it does become easier, and works every time.