A huge issue for TfL is how to cool the deep tubes as they get hotter all the time, and decades of heat generated by motors, other equipment and people has accrued in the tunnels and the surrounding substrate, meaning dissipating it becomes ever more difficult. Obviously, air-conditioning only worsens the problem outside the trains, an issue that New York suffered/s from with their air-con trains and stations so hot the chewing gum on the ground never sets! The London tube is uncomfortably hot sometimes in winter as people are wearing warmer clothes, and often almost unbearable in summer. However, all tube stock has heating because large stretches of route are above ground.
Surely this doesn't include the Victoria line which is exclusively below ground. And even in January very warm.