Mains supply in Poland: 230 V, 50 Hz, sockets of Types C and E. At 230 V the supply matches what UK and European appliances expect, so no converter is needed.
Handy for multi-stop trips: Monaco, Belgium, France and Slovakia use the same plug types. Nearby but different: Malta, Cyprus and the United Kingdom – an adapter for Poland will not fit there. Across Europe this setup is fairly rare: 6 of 50 countries.
Protective earth
Grounded and ungrounded sockets may occur
Not every socket in Poland is earthed: Type E yes, Type C no. Class I appliances belong in an earthed outlet.