The 1975 Channel Tunnel project was cancelled when the first two TBMs were installed and ready to go (a trial bore had started).
However, unlike HS2, the sunk cost up to that point would have been relatively small with an undersea route and little land purchase or property demolition needed.
Wiki reports the cancellation cost as £17 million, which was chickenfeed!
The (Labour) government decided it couldn't afford to build Concorde, a new London Airport and the Channel Tunnel at the same time*.
CT costs were rising out of control as well, which was the last straw.
I don't get the feeling we are in the same position with HS2 - there's too much cross-party political capital invested in it.
*Concorde came and went, and we are still waiting for the 3rd London Airport.
Concorde couldn't be cancelled because we had signed up not to, in order to prevent the French doing it to us