I Googled your organization about fox in the UK. http://www.nfws.org.uk/mange/ They offer free homeopathics, but appreciate donations.
They recommend (Arsenicum Alb. & Sulphur 3 0 c ) which are homoeopathic remedies found in the Vetionx product DERMA-IONX. They state:
" Since the remedy is made up from natural ingredients there is no fear of overdosing and nothing to worry about if a non-infected fox or other animal or bird takes the treatment. To avoid the local cat population stealing the treatment meant for the fox, we suggest that it is administered via a honey or jam sandwich; foxes love sweet things - cats donít! Since foxes are instinctively selfish with their food, if you have more than one fox visiting your property suffering from mange we recommend that the food is scattered widely rather than being placed in one spot. This will then ensure that more than one fox feeding at the same time can do so without having to be nose to nose with each other. Dosage: We recommend that four drops of the treatment should be administered for each fox daily. Cut the sandwich/es into 12-15 tiny pieces and put a drop on each tiny piece, and then scatter them around the area you see the foxes. After roughly three/four weeks of treatment you should notice the fox/es scratching and biting less, and eventually you will see the new hair growing back. Once the hair has started to grow back you can stop treating. "
DERMA-IONX contains a variety of remedies in a oligotherapeutic mineral base that address many different skin conditions. I hope you are able to help the fox in your area.