My missus got the train to London from Wakefield last Tuesday. 1hr 57minutes, bang on time, table seat booked, not overcrowded, scan a QR code and order your breakfast straight to the table. Seamless. Tell me what difference £100 billion will make to that journey.
Nothing as it doesn't go that far north

What it will do is allow more freight on the west coast mainline (which is saturated at the moment)
It can't free up any slots on WCML without impacting existing passenger services, as there are so few express services - and even if you could remove a few express services from WCML, that's not enough to create a freight slot.
Additionally, WCML is not at capacity, hence Network Rail recently suddenly finding 400 free slots on it, but only after the current government committed to HS2 Phase 1....
As you know, my office is in central Brum, and most of the hardware I look after is in Londonium, so I do get to use both existing lines regularly. The WCML express trains are under utilised, because hardly anyone travels from New St to Euston. The slower trains on WCML tend to be much busier, as the vast majority get off en-route, usually around Coventry, MK or Leighton Buzzoff from trains from Brum, and Watford, Hemel, Tring, Leighton Buzzoff and MK for London trains.
Chiltern tend to run 2, 3 and 4 carriage DMUs, but all stations can support 8 carriage trains, so no shortage of capacity there either. Just the trains are old, unreliable, and always late
