Yes, but even then the BCRs are not very good.
The
BCR of Phase 2a in 2015 was ~1.3 (page 19/38) after accounting for additional construction inflation, putting it in the
"low" value for money category. There has been substantial further cost growth since then.
But the proposed bottleneck works aren't actually
necessary.
HS2 and the wider rail industry may
want them, but are not strictly necessary.
At the time of its cancellation HS2
admitted to costs of £5-7bn for
Phase 2A in 2019 prices.
Given the history of major cost growth and accusations of a culture of cover-ups reported by various media outlets, it is unlikely it can be delivered within that envelope, which in any case is ~£6.1-8.6bn today.
Add to that the cost of the delay/disruption and standing up a new delivery body, and you will be lucky to get it for £10bn.
The cost of Phase 1 isn't really important, we must always be careful to avoid the sunk cost fallacy.