Unfortunately you fail to appreciate the point being made by Sir, and in my view you represent a common issue with HR departments, i.e. the failure to communicate with management.
We have no idea of the operational demands that may have to be managed in this situation and for any hr manager to assume, well we know what Ass U Me makes!!
That said, to deny leave in this situation is ridiculous as the problem is not the making of the employee but (it appears) the failure of hr to check with management if this is acceptable.