pls dont rely on the RSPCA, we had a bad experience with them, and the baby bird in our care had to wait over 17 hrs for them to arrive by which time he got weak and died in their van on the way to the vets. they didnt even take him to a local vet, they drove him for over three quaters of an hour, it was just all too long for him. he was newly hatched and needed someone experienced to feed him ( we didnt know how to

) he was healthy all throughout the time we had him, tweeting away.
also be aware that a vet may well try and put the fox down, many vets do this with wildlife. so as mentioned above by RangerSarah,pls insist they dont put it down.
i know of a site you can find info on and depending where you live they will come out and help
NFWS Fox Rescue and Sarcoptic Mange Information Site-We also supply mange treatment for foxes suffering from Sarcoptic Mange known also as Canine Mange and Fox Mange
whishing you best of luck