I think I remember reading back in the day that they buried/parked the machines near the center of the Channel where the French side and the English side met at the middle.
EDIT: Dogma replied on this as I was typing
"In December 1990, the French and British TBMs [tunnel boring machines] met in the middle and completed the Channel Service Tunnel bore. In all of the tunnels the French TBM was dismantled while the U.K. TBM was turned aside and buried"
I wonder if it was simply time management to have both sides meet at the middle or politics/symbolism. The former?
That tunnel is over a kilometre long, and they met in the middle of a hill!