I'm not fully conversant with the AIP but is there a statement that says once airborne and the weather at destination is below alternate minima but above approach minima, an alternate is required. I don't recall a statement that says this. It sounds like good practice, except for having to comply with a ceiling when an ILS is available and if your destination is a place like ADL with no available alternates and the weather changes, you go from being legally dispatched to illegally airborne and there is nothing that the operator could have done.