On the flip side, if one of these robotic rodents = one real life mouse or rat saved:
One female mouse can have up to 10 litters in a year, with numbers ranging typically from 5-14 pups each (average number that survive is closer to 7 or 8, but we'll go with 8)
Numbers are very similar for rats, so we'll just go with the above numbers.
Since obviously predatory species aren't taking out every single rodent ever, we can assume that each of these rodents has the capacity to become a breeding pair with a rodent that would otherwise not be eaten.
So if these robots fed the foxes of the UK and totaled to 3.4 million robots in a year, and each robot is equal to one mouse or rat saved...
Roughly 8 pups per litter x 10 litters per year = 80 new mice added to the population per breeding adult mouse per year
3.4 million rats/mice saved from being eaten by foxes x 80 new mice added to the population per breeding adult mouse saved = 272,000,000 (272 million) new mice/rats added to the population for every year this project is a success.
And this is just the number OP stated for foxes in the UK alone. There are many other predatory species living in the UK (both native such as owls and other raptors, and invasive such as domestic and feral cats), some of which rely solely on mice or rats. The amount of mice and rats consumed total by every predatory species in the UK alone is probably in the mid-to-high millions, and as I'm sure everyone knows, there are still plenty of mice and rats in the country. If the predators of the UK would be made to instead start hunting robotic mice and rats, we're talking potentially many multiple billions of new mice and rats added to the breeding population every year in the UK alone.
That's not good for people, since wild mice and rats can carry fatal diseases, but it's also not good for the wild mice and rats as they suddenly need to compete even more heavily with each other for resources and have more chance of spreading diseases and parasites to each other. Not to mention, with all of these new rodents needing to eat, we would see significantly more crop damage and loss, and there would be significant damage and loss to native plant species as well. Also, this should go without saying, but predatory species typically physically are incapable of surviving solely on plant matter, especially when relegated plant matter found in their natural habitats.
This entire project, if ever taken seriously, genuinely attempted to be implemented, and even in some slight way were to become successful, would be a disaster and a death sentence to the whole of nature and ecology as we know it.