I work in the legal profession in the UK, so at present, I don't know whether there are opportunities for UK lawyers, either working in Polish firms, european firms based in Poland or alternatively in-house for Polish companies expanding into wider Europe.
No chance at this firm; When they want a UK-qualified lawyer, they recruit a Pole who is a UK-qualified lawyer as well as Polish-qualified. Or rather they recruit a Pole who is a US-qualified lawyer and has taken the conversion course to become UK-qualified.
I do know a few British lawyers working Warsaw, well, a couple anyway, but they tend to be senior associate level and transferred within the firm. Your best bet is probably going to be teaching legal English.