What are the options for sending parcels/boxes of books 5-10-20kg?
Some Polish people that I know, send parcels/boxes back to Poland with the help of a Polish church. There is a collection once a week and the boxes etc, are then all sent together and takes about 1 week to get to various cities in PL. The cost is £1 per kg, quite cheap.
It might be worth finding out if there is a collection service like this near you.