Check out these documents:
UK/POLAND DOUBLE TAXATION CONVENTION
hmrc.gov.uk/taxtreaties/in-force/uk-poland-dtc.pdf
CONVENTION BETWEEN REPUBLIC OF POLAND AND THE UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND FOR THE AVOIDANCE OF DOUBLE TAXATION AND THE PREVENTION OF FISCAL EVASION WITH RESPECT TO TAXES ON INCOME AND ON CAPITAL GAINS
finanse.mf.gov.pl/documents/766655/1155165/20060720_WlkBrytania_konwencja_tekst_ang.pdf