I don’t know. But at an educated guess, I would imagine the consultation, numerous surveys including geotechnical (the ground isn’t good there, as Cambridgeshire found out to their immense cost) the outline design that results from both, all the necessary reports that need doing before a consents application including the Environmental Impact Assessment, ecological and traffic surveys, land referencing; detailed estimates, schedules and risk assessments; timetable modelling, and possibly initial procurement stages for the main contracts, and a bit of land purchase if that can be done by negotiation rather than compulsion.
Be under no illusion, this is potentially a big job.