UK Rail and Trains

My understanding is that this will require new designing and consulting stages of phase 2. We have already spent about £2billion on Phase 2 which is likely not recoverable so you would need to respend at least a significant fraction of that on new design consultation and lawyers etc. So any cost savings you expect from different design requirements would need to be much greater than that (probably around 3-4% of total cost).

Yes slower services allows more flexibility with alignments but it comes at a cost of larger fleet sizes and likely more warehousing requirements(unless you reduce the passenger capacity to correspond). Speed was looked at in the original plans and found that reducing the speed somewhat did not reduce overall costs that much but did reduce the outcomes quite a lot.

The biggest problem is in the way costs have been amalgamated and communicated. HS2 had lumped in some really major project works that needed to happen anyway (notably rebuilding Euston that is currently not for for purpose for current passenger numbers) alongside at least two new stations to facilitate interconnections with the rest of the network. In other countries thought would come under separate budget lines and not look like one project.

The other big cost factor for HS2 was simply to demand more from it. We required it to be incredibly good at avoiding as much ecological disruption as possible and that meant more expensive tunnelling. It would have been the UKs only climate resilient line in the country partly as a result. So as another commenter said if its cheaper (which I would stake money it won't be significantly) it will be at the cost of much less care towards the environment and offering a much less future proof outcome. If we wish to meet climate obligations we need massive increases in rail usage and that only begins to be possible if you free up this scale of capacity from the rest of the line.

The other thing to say is the cost is a bit if a fiction in itself. The cost is paid for by borrowing against future revenues of the service so to downgrade the service to save 1% of cost and you potentially downgrade the return even more which means you could actually cost the treasury more. This isn't money that is available for anything else despite how its been reported.