The advice I've always had was that cats and foxes were an equal match for each other but foxes were usually more cowardly so I never had any qualms in letting our cats out at night. My elderly cat ambushed a fox in our garden by hiding in a bush and jumping out at him with a swipe and he ran off. She was bitten once however and vet concluded it was either an enormous dog or a fox due to the predicted jaw dimensios. However they are becoming more brave and there are so many proven incidents around of their attacking young children, so things are clearly changing.

I dont see how cats can be protected from foxes however. They are a law onto themselves however it seems that some sort of deterrent is needed, but not sure what.