HS2 only got permission to be built from parliament and royal assent with the term "Full Y-shaped network" applied, as far as I am concerned, HS2 ltd are in breach of contract, and planning permission if they build the network without the Eastern Leg.
Each Phase of HS2 has a business case that stands on its own merit, and does not require subsequent Phases to be built in order to stand up (but the Bills will describe how future phases are supported).
E.g. Phase 1 as an entity has a business case.
Phase 2a has a business case as an extension of Phase 1
Etc.
