Property purchased prior to 2007 and held for a full 5 calendar years is not subject to any Polish capital gains tax. Property held for less than 5 full calendar years is subject to a 10% flat tax on the net proceeds of the sale. This is not a true CGT, it is 10% of the proceeds, regardless of profit.
There are ways to avoid the tax by re-investing the proceeds in Poland. Consult a lawyer before selling.
Property purchased from January 1, 2007 is subject to a 19% CGT on profit. This can be avoided if the property is used as your registered primary residence for 1 year (bought in 2007) or 2 years (bought Jan 1, 2008 forward)
UK Resident - I believe you are correct. If you are tax resident in the UK, you are taxed on your global income. Any taxes paid to Poland should offset the UK tax.