gives ranges of 26-136 zł for economy mail, 28-165 zł for priority mail, and 34-171 zł for express mail for packages weighing 1-20 kg. This translates to anything from £5 to £30 (~5.6 zł to £1). As I live in the States and don’t know anything about Royal Mail, you should check the site to see if your particular parcel would be cheaper to mail from Poland or from the UK.

I’ve cited the postage calculation site if you want to go ahead and use it yourself. It’s initially going to be in Polish - click on “Paczka postowa” under the second column (”Usługi zagraniczne”) and it’ll open a new window. Click on the British flag on the upper left corner, and you’re good to go.

Hope this helps!