TSEM9862 - Property held jointly by married couples or civil partners: Form 17 rule - strict time limit
The Form 17 declaration must be given to HMRC within 60 days of the date of the declaration.
The declaration time limit of 60 days must be enforced strictly, s837 3(b) ITA2007 . There is no power to extend it.
Since difficulties may arise many years after the declaration - perhaps if the couple split up - HMRC must make sure the notice of declaration Form 17, is valid at the outset.