I have been looking into different ways of sending parcels from the UK to Poland and found Meest UK. One thing I liked about their service is that you can sort out most of the arrangements online, without having to make a special trip just to enquire about prices and delivery options.
They offer both air and ground transportation, with delivery to the recipient's address in Poland. You can calculate the estimated cost on their website, register your package and choose a convenient way to hand it over. There is also a tracking facility, which is useful when you want to know how your parcel is progressing.
I would suggest checking their recommendations before packing, especially the information about dimensions, customs and insurance. The FAQ section covers quite a few practical questions too.
Overall, Meest looks like a sensible option for anyone arranging deliveries between Britain and Poland. Here is their official page with all the details: www.uk.meest.com/services/international/delivery-to-poland