There's an authoritative fact sheet about ringworm control in a shelter setting on the UC Davis Veterinary School's Shelter Medicine site. Be aware that it's based in the US, so some of the veterinary preparations mentioned may not be available in the UK. However it does give a very clear explanation of exactly why ringworm is so difficult to deal with in a situation where there is continual pressure to take in more animals who will suffer or die if no space is found to house them.
