NIM30013 | Special Cases: Married Women: Payment of voluntary contributions
From HM Revenue & Customs · National Insurance Manual
Regulation 132, Social Security (Contributions) Regulations 2001
When a woman cancels her election she can subsequently pay Class 3 contributions for the whole of the year in which full liability started even if there is full liability for only part of the year.
However, Class 3 contributions cannot be accepted;
if they will not count for or increase benefit entitlement; or
for any year throughout which the woman had reduced liability
For further information about Class 3 NICs see NIM25000