[pip08456]

Gumtree is the first that comes to mind but there's a few.

http://gumtree.com/
http://tapthat.co.uk/
http://hallolondon.co.uk/
http://london.adzooks.co.uk/
http://vivastreet.co.uk/
http://adoos.co.uk/
http://freeukclassifieds.co.uk/
http://ad-mart.co.uk/
http://www.loot.com/
http://uk.freeads.net/
http://www.adoos.co.uk/
http://www.friday-ad.co.uk/

I suppose it depends on what you are selling and how much of it.