LEP was annihilating electrons with positrons; LHC does not, as it only accelerates protons. FCC will have the option to do both: the plan includes two separate pipes in the same tunnel, one for protons, one for electrons.
This however does not increase the collision energy significantly: a single electron-positron pair has the (rest) energy of ~1MeV -- this is what is released during the annihilation, but the energies we want to go to at the next collider go up to 10TeV, so 10^7 times larger.
This however does not increase the collision energy significantly: a single electron-positron pair has the (rest) energy of ~1MeV -- this is what is released during the annihilation, but the energies we want to go to at the next collider go up to 10TeV, so 10^7 times larger.