correc these omissions at a later point. Extending platforms on an
existing line or adding a station will cause seemingly endless WCML
style disruptions and overruns. Is this worth it?

No way will they worry about that! The idea will be to cut as much as
possible while still allowing Boris to claim that the project has been
saved thanks to his lobbying, and thus allow him to be reelected.
Doesn't matter if problems emerge in the 2020s, or even on completion:
Cameron can simply point out to Boris that by that stage, "I'll be
gone, you'll be gone".