TSEM9864 | Property held jointly by married couples or civil partners: form 17 rule - when a declaration stops
From HM Revenue & Customs · Trusts, Settlements and Estates Manual
The split of income for tax purposes produced by a valid declaration goes on running for all later years without any further action until one of the following events happens
one spouse or civil partner dies
the couple separate permanently
the couple divorce or the civil partnership is dissolved (where the couple have not already separated permanently)
the beneficial interest of either spouse or civil partner in either the property or the income it produces changes; for example, this can happen if one spouse or civil partner transfers any part of his/her beneficial interest to the other or to a third party.
The couple cannot simply choose to end the split of income which results from a declaration; it goes on running until one of the four events listed above occurs. But even the smallest change of interest (4 above) stops the declaration running. The standard 50/50 rule then applies again unless the couple make a fresh declaration.