Half-Price books is in the US, but there is probably a British equivalent.

You could offer them as a lot on eBay, get rid of them in one fell swoop. Be sure you package and weigh them FIRST so you know how much to charge for shipping. And if you're offering to sell internationally, you might suggest that bidders contact you and you'll research the rate and let them know.
List them cheap, maybe 25 - 50 cents a book to start, and go from there.

SRS