The other issue is that if the Queen lives to a very grand age and (say) Camilla was unwell, Charles might 'disclaim' before he gets to be king. Presumably, that also requires an act of parliament.
Her Majesty, God Bless Her, shows every sign of living to a great age and it is not beyond the bounds of possibility that She might, like my own grandmother, outlive Her elder offspring. An outcome that would solve any succession problems.