Cats and foxes generally avoid each other. If there were to be a confrontation then, in most cases, a fox would come off worse.
The only time there could be a problem is when there are fox cubs around, since the fox may be protective of them. In such cases, we would advise people to keep their cats indoors at night until the cubs have grown up and the family has moved away.
For more information about foxes, read the RSPCA’s Living with foxes (PDF 72KB) information sheet and visiting the foxes page of this website.
Other useful information: