Culex pipiens exists as two ecological forms. The typical form feeds mainly on birds, requires a blood meal before its first egg batch, mates in open swarms and passes the winter as a dormant adult. The form called molestus does none of these things: it can lay a first batch without feeding, mates in confined space, stays active all year and readily bites mammals including humans.
Genetic work in the late 1990s on mosquitoes from separate Underground lines found them to be differentiated from surface populations, and the animal entered popular science as a species evolved in a century inside a railway.
That story is the part to hold loosely. Later analyses indicate that molestus is an old form, probably of Mediterranean or Middle Eastern origin, associated with human structures long before the tunnels were dug. What London provided was a warm, enclosed habitat where an existing form could persist, not necessarily a birthplace.