I have a feeling too, that at once time, you needed a certificate to prove that you are are not married and are free to marry but ask a priest who will conduct the ceremony for confirmation.
This is correct and you can get this from your local town hall in the UK, but there is no ready document for this and you will have to get someone to give you a sworn letter stating that you have never been married before.
One word of warning, on my sworn letter it said, "to the best of my knowledge", and when I had it translated (by a sworn translator), they would not except it as it was not 100% sure that I haven't been married before, so I went back to the translator and asked them to use a bit of artistic licence and change it, omitting the problematic phrase, then everything was fine.
Also take a look at this post:
https://polishforums.com/archives/2005-2009/life/poland-english-man-getting-married-13031/