Big fans to pull surface air down into the tubes when the tubes are warmer than surface ambient ? Cool the tunnels, warm the surface.
Just about every abandoned station, elevator shaft, and maintenance tunnel on the network is already fitted out with huge fans where possible.
TfL also runs a semi-continuous works project that looks a custom and novel one-off cooling solutions that can be retrofitted into whatever space is left. Including complicated hydronic systems that pump around huge quantities of water where the infrastructure allows for it.