I think the Bucks appeal form is worded as it is because they don't want the head's statement coming in separately. In any case, they won't arrange your appeal until they have both bits of paper, or a declaration that you don't want the head's statement admitted as evidence! [I wouldn't recommend the latter!]
If you apply late, you must explain why. But I assure you this particular explanation will be accepted.
Of course, if the head is very
late, then I would suggest a phone call to the appeals office to check what you should do (01296 383695/383677/383384).