shipping/mailing books from UK to England

After a year of sabbatical in Oxford, I am (sadly) getting ready to go back home.  In my time here I have accumulated some books that I would like to send home---books that I need for my work and books I just like to have.  What is the best way to do this?  Books are heavy, I know, and I have enough to fill two medium sized boxes.  On the other hand, I don't need them quickly, although it seems as if the "slow boat" options aren't so readily available as they once were.  I would appreciate any advice anyone might have, and I apologize if I have missed something obvious--I did search the list before posting.

thanks in advance.

you need to clarify - are you shipping back to the US? because that's not what your subject reads! Royal Mail does do sea mail, but you could also look into excess baggage companies. If indeed you are shipping to the US, you should look at the Dive Into America forum - they may have more ready answers than those on here who are generally moving US to UK!

Oh, I'm sorry for mis-typing my topic heading!   I will try the forum you suggest.  Thanks,