Certificates of No Impediment (CNI) to Marriage
If you're going overseas to get married, you may need a Certificate of No Impediment to Marriage (CNI). Authorities overseas may ask for a CNI to prove you're free to marry.
https://www.smartraveller.gov.au/consular-services/notarial-...

Details
This notice and affirmation is for British nationals getting married or registering a civil partnership in Poland where you can apply for a certificate of no impediment (CNI) by post.
https://www.gov.uk/government/publications/poland-certificat...

Marrying in Poland, the Czech Republic, Lithuania and Ukraine


Issue of Certificate of No Impediment to Marriage (CNI)


The law in some countries (including Poland, the Czech Republic and Lithuania) requires that foreigners wishing to marry in that country must obtain a certificate stating that there are no impediments to their marriage. Impediments to marriage include being already married, marrying a minor, entering into a forced marriage or marrying a close relative. The certificate is known as a Certificate of No Impediment to Marriage (CNI).

https://poland.embassy.gov.au/wsaw/marrrying.html