Sycorax, the 17th Moon of Uranus, is a ball of rock and ice some 190
km in diameter. It is believed to be a "captured Kuiper Belt object".
Kuiper Belt objects are small planetoids beyond the orbit of Neptune
(between 30 and 50 AU from the Sun).
The presence of the Sycorax using rock-based vessels in the solar
system suggests to me that they may come from the outer solar system,
possibly from within the Kuiper Belt.
Further, the reference to the Sycorax ability to summon a fleet (and
there's nothing to suggest they have FTL capacity) suggests that
similar ships might exist nearby.
Imagine the repercussions of the Sycorax living so close (in galactic
terms) to Earth.
Then there's Mondas.
I struggle to understand how the planet could ever travel beyond the
Kuiper Belt before returning to Earth, especially when there are a
host of occupied trans-neptunian worlds from Triton (home of the Waro)
to Xena and Gabrielle, to Vulcan, Desperus ("the prison planet of the
solar system"), Voga/Neo-phobus (from Revenge of the Cybermen) and
Yuggoth (a Darkling colony).
So there's plenty for the Cybermen to mine and interact with at the
outer reaches of the solar system, and good reason for them and the
Sycorax to be arch enemies well-suited to going to war with each other.