If the dining chairs have upholstered seats nobody can legally sell them unless they've got a fire safety label. If they're not illegal (and assuming that it's not the same charity which you've already tried) the British Heart Foundation might accept your donation. (They have specialist 'Furniture & Electrical Stores'):
http://www.bhf.org.uk...-to-donate-goods.aspx
Many other charities, who might be interested, are only local. (For instance, we've got the 'Ipswich Furniture Project' in this area). Your local library might have details of something near to you.
Chris