My fiancé says that in a polish church wedding
Depends on the church in question. For most mainstream churches operating in Poland, a church marriage is automatically recognized by the state without the need for a separate civil ceremony.
"In Poland, the marriage rite conducted according to the traditions of some religions can be recognized and registered by the civil registry office (USC) without having to enter into a separate civil marriage. This is the so-called concordat marriage. The following churches and religious associations can conduct valid concordat marriages:
1. The Catholic Church, including the Roman Catholic, Greek Orthodox, Uniate and Neo-Uniate and Armenian Churches,
2. The Polish Autocephalous Orthodox Church,
3. The Lutheran Church,
4. The Reformed Church,
5. The Evangelical Methodist Church in Poland,
6. The Baptist Church,
7. The Seventh Day Adventist Church,
8. The Mariavite Church,
9. The Pentecostal Church,
10. The Polish Catholic Church,
11. The Union of Jewish Communes."
As for a marriage certificate, yes, of course you get one. And the UK is bound by treaty to recognize it as valid.