Unfortunately, I am only being realistic when I say that HS2 Phase 1 is going to be a disaster on all fronts.
I will be realistic too.
Upgrading the Chiltern to 220kph means completely rebuilding it from the ground up. Every station would have to be rebuilt too, as you can’t have platforms open adjacent to lines at more than 200kph. There would need to be long sections of 4 track also as trains at 220kph would catch up stopping services much more quickly, and thus need to overtake.
Further realism - Oxford - Bicester was closed for 18 months to rebuild 11 miles of railway to 100mph standard, without electrification.
Some more - because of the works you propose, you would need a Development Consent Order (DCO), which means going through all the consultation and public inquiries needed for a new railway along its entire length.
A bit more - because the Leamington - Birmingham corridor is effectively full, you would need to build some new tracks in to Birmingham too, unless you were happy with the existing frequency of service as the result of the proposed works.
Essentially, then, what you are proposing is constructing a brand new 2 track railway from NW London through the Chilterns to Birmingham. Sounds familiar? Except it would be built on the route of an already busy railway, which would need closing for at least half a decade (conservatively). And then not offering an increase in service.
I’m afraid it doesn’t sound such a good investment to me.
That’s all before I ask about where underneath Marylebone these extra platforms are going. Through the Bakerloo line or under it? And if the latter, where are the accesses going to go - through the Bakerloo line or around it? And if the latter, which of the listed buildings are you going to knock down in Marylebone to provide the street level access?