When this one comes up I always point out it would be better to not bother with HS2 and spend the money on making it easier for people not to have to travel around.
"There is no available capacity without significantly impacting performance and causing a reduction in timetable resilience due to the resulting requirement for successive services to run on minimum headway"
https://www.orr.gov.uk/sites/default/files/om/wcml-capacity-...