I've no personal knowledge of this, but some have an online presence & therefore presumably send books out regularly. One list I found:
Postage rates are due to increase next week, but these links are to the services via the post office for an idea of their prices:
Why don't you just get a box, pack them up and take them to a post office and post them to yourself?
Should you decide to parcel the books yourself and post them, this site tells you how much you might have to pay. A kilogram is about 2.2 lbs
There are other couriers besides Parcelforce. Personally, I would leave it in the hands of the bookseller.