There's a Polish courier company called Eurospedycja, based in Ireland, who can ship from the UK to Poland. If you just want to send stuff like books, cds, clothes, general bits and pieces they work out very reasonable. I've used them and found them very reliable. They also do regular removals starting at 246 euros.
eurospedycja.com/en/offer/pricelist
eurospedycja.com/en/offer/removals