In the past, there have been many different routes proposed for new tube lines, and each new line which was proposed came with multiple different proposed routes at different stages before construction began.
It is not difficult to see how a novelist, writing about the tube network in the 1980s, at the time when planning was in progress for the new Jubilee Line, could have been writing at a time when the route, which was fairly uncertain at many stages of the planning, was intended to pass much closer to Downing Street.
The novel is set 15 years after 1982, the date of writing: it is perfectly reasonable to take the view that a writer in 1982 could legitimately have believed, given the uncertainties involved, that a decade and more later the new Jubilee line would lie quite close to Downing Street. He was speculating about a largely unknown future.
The proposed route which would have routed the new line from Westminster tube station to the existing station at Picadilly Circus, instead of to the neighbouring station at Green Park to which it now runs, would have in fact taken the Jubilee Line beneath Downing Street.
One point I can definitely clear up. The use of the name 'Victoria Line'. Many of the initial proposals were to build the new line, today called the Jubilee Line, as a simple extension of the Victoria Line. It was originally going to be much shorter: it grew in the planning, until eventually it became a completely new line: but in 1979 it started out as a short extension to the existing Victoria Line. It was a long way down the road in the construction phase before the belated decision was taken to name it the Jubilee Line.