Because there are so many things that can cause your fruit tree to die, and voles would be a very rare reason in England. Voles do eat succulent roots, like bulbs and potatoes, but they don't normally eat woody tree roots. When they do damage fruit trees, try do it by ring-barking. But that doesn't happen often in England, if your tree had been ringbarked by small animals, then here in England it would be much more likely to be rabbits. Voles is much more of a garden nuisance in America, where they will rearrange all your garden bulbs, and eat most of them, most years.

Voles nevertheless are extremely common in England, they are basically everywhere if you are anywhere near fields or open country. They form 90% of owls' diet, they are that common. I caught two voles in traps in our kitchen just the last two weeks. I've seen them in the compost bin. I've seen them pinching the seeds from the bird feeder that the birds drop. I've found their nests in the shed and the garage. I regularly hear owls, so they must be hunting the voles hereabouts. But they have never damaged any of our trees.