The RSPCA website has a leaflet about foxes, what to feed them and what you can catch from them it is at this link (sorry not a clickie!)
http://www.rspca.org.uk/servlet/BlobServer?blobtable=RSPCABlob&blobcol=urlblob&blobkey=id&blobwhere=1024473253585&blobheader=application/pdf
They will euathanase if the animal is deemed to be suffering and is beyond redemption, but when I worked there as a teenager, we had several foxes and fox cubs brought in that were all tended back to good health and released into the wild.
Cheers, Lucy