It is very difficult to keep a fox out of a garden, as they can climb very high fences. Our foxes visit regularly, but haven't tried to break into the hutches, and now I have sheds, which are even stronger than the hutches were. The bunnies don't seem to be too bothered by the foxes, they thump a few times and then go back to normal as soon as the fox has moved on. Ferrets are natural enemies of rabbits, too, and they live in your garden already. Just make sure the bunnies are in a secure hutch/run/shed.