The problem with HS2 is there isn't a lot you can descope without effectively cancelling the project. Dropping the speed to 300kph wouldn't save that much (if it's not there already) and the stations themselves are relatively small proportion of the overall budget. The only big saving you could make would be to end the line short of the urban areas, but then the whole point of HS2 is to ease capacity on the classic lines to major urban areas, so that doesn't make any sense either.
HS2 Phase 1 construction is still at the early stages, and is both labour- and energy-intensive (ie subject to maximum inflation pressure).
I think I saw that 28,000 jobs were now dependent on HS2 contracts.
Just as the Channel Tunnel had to be completed at whatever the contractors' cost, HS2 is now in the same state - once started you have to keep feeding the monster.
The railway contracts have not been let yet (track, signalling, OHLE) - maybe there is some descoping potential there.
Rolling stock is also uncontracted (although there is a preferred supplier).