My wife and I were both born and lived in Poland. We now live in the U.K and have since had a baby girl in the U.K. What will my baby girl be? Polish or British

Born to polish parents, she is polish as well.

UK Border Agency | Children born in the United Kingdom

I found this which you may find helpful, at the end of the page section 3, is probably what your looking for.

Hi red, good website, but I think the child would have dual citizenship, same as my son being born in US to canadian parents. He has two passports US and CDN so he has dual citizenship. Basically means that at the age if majority they have a choice.