I have no idea how the initial investment was financed. Were there private investors?
What I am surmising is that there was no lump sum originally allocated to a government account called 'HS2 finance' (or something similar). All the state/government committed to was paying the bills for HS2 when presented to them. I don't know if there was a limit imposed beyond which they would stop paying; it doesn't appear that there was.
So, in that case, unlike a household account in which money were put aside for, say, 'home improvement', the money for HS2 most probably wasn't 'put aside. There probably isn't an actual balance saved by cancelling it that can be spent elsewhere, just a theoretical balance.
I might be wrong , this is just a strong suspicion on my part 