London Bridge

Certainly the death penalty if you can be sure 100% they are terrorists; so they have committed and act of terrorism and been caught in the act, not planned, not schemed, not encouraged, but actually committed.
If you want to lock them up and throw away the key with no hope of release then that is inhuman. Indefinite sentences for public protection have recently been abolished for this reason and the UK Supreme Court would almost certainly rule against them.
Indefinite sentences also make prisons more violent and harder to manage, as these prisoners have nothing to lose if they attack fellow inmates or officers. If you want to see the prisoners take on them read "A Sense of Freedom" by Jimmy Boyle