If property is a family home, and in the sole name of one spouse, it cannot be mortgaged or sold without the consent of the (non-owning) spouse. Obviously, where it is in joint names, both spouses have to sign anyway.
The land registry will not register a property transaction of any sort without evidence that either:
a. It is a family home and the necessary consent has been given OR
b It is not a family home, so consent is not needed.
The declaration you signed provides such evidence.