Law / My bf's visa expired and we want to marry in the UK [9]

Hey guys, maybe some of you have experiences like this and hopefully will give me some answers.

My boyfriend is indian living in the U.K. for last 7 years. I live in Poland and don't really wanna move, at least not for long. His visa expired 3 years ago but he stayed anyway. Of course we'd like to be together but it's getting more and more difficult with brexit coming. We decided to get married, guess it's the only way for us to stay together. Of course it's gonna have to happen in the U.K. cause he can't go anywhere. But eventually I'd like him to stay in Poland.

Yeah, we went to a lawyer but we didn't get all the answers we need. Does any of you know a case like this? Or maybe know how it works and can tell me step by step what we need to do? And is it even possible for us to go and live in Poland together eventually?

Thanks guys for help.