I am looking to buy my first property in this country
When you say "this country", do you mean the UK?
how would I start going about using the Polish property for equity?
You most probably can't. In theory it might just be possible but in reality no UK bank is going to be interested: it just is not what they do and they don't know about the legalities, which means they won't want to know.
Can you not sell the Polish property?