It all depends on what type of music it is. If it is mostly the type of "pop" that gets (or was) played regularly on UK radio, you might as well give it to charity. If it is more specialised, e.g. blues, country, prog., etc, then you might get a couple of pounds per CD if you offer it to a specialised second hand dealer, or try to sell it on ebay.
Just a couple of examples of the prices they can fetch:
Find great deals on eBay for Blue Cheer CD. Shop with confidence.
www.ebay.co.uk
or
Find great deals on eBay for Flower Kings CD. Shop with confidence.
www.ebay.co.uk