My understanding is that you cannot undergo a second initial class 1 if you have at any time held a class 1. Even after Brexit you will have held an EASA class 1 so you cant redo an initial. You can renew. Transferring your medical before Brexit is straight forward. Allowing it to lapse and then trying to renew based on records from a non EASA country, in a country that often has its own interpretation of EU law, seems full of risk. So I would renew before 29 March. Whether you do so in Poland or the UK depends on whether you think Poland offers any advantages to you.