Property held jointly by married couples or civil partners: Form 17 rule: Who can make a declaration?
A form 17 declaration (ITA/S837) to opt for an unequal split of income for tax purposes applies only to property that would otherwise be subject to the 50/50 rule (ITA/S836).
A married couple or civil partners who have separated would not be subject to the 50/50 rule, as it applies only to couples living together (TSEM9810). They will be taxed on their actual entitlement in any event, and so cannot make a form 17 declaration.
Individuals other than spouses or civil partners cannot make a form 17 declaration, for example siblings. The 50/50 rule does not apply to them. Income is attributable to them on the basis of their entitlement.