
I am surprised at your UK foxes, they appear to do their business right out in the open where your dogs can get at it. There are lots of the lovely little critters here but I have never encountered their leavings. Or maybe I'm just lucky that Jet is not interested in that particular smell, LOL. However, the late winter air, when it is mating season, is full of the smell of fox, which is remarkably like skunk. So, assuming their poopers also smell like skunk I would suggest a specially formulated shampoo for removing odours, for skunks if you can find it. I've used both tomato juice and the special shampoo, for skunk spray, and find the shampoo to be much more effective. And needs to be applied with warm water.
Edited to add, won't turn your white dog pink either. Ever seen a pink ESS?