I am GB citizen working in Poland. Can I deduct costs related with the work I am performing here (some incurred in GB though)?

The tax deductible costs are all costs incurred with the purpose of generating revenue, retaining or protecting sources of revenue, except for the costs which are explicitly listed in the PIT Act as not deductible.

The costs can be direct and other costs as described in the PIT Act.

Generally, the direct costs are deductible in the tax year in which the related revenue was earned. Other costs are deductible on the date they were incurred.

At the event the tax deductible costs were incurred in foreign currencies, these should be converted into PLN at the average exchange rates announced by the National Bank of Poland valid for the last working day preceding the day the costs were incurred. The exchange rate differences shall respectively increase the revenues as foreign exchange gains or increase the tax deductible costs as foreign exchange losses.