Earlier quoted context omitted.
This. London's tube system is essentially stopped a day or so a year due to strikes, which is pretty dramatic for a city of 10 million people who rely on it.
Most strikes affecting the underground have not been just drivers, so making the system driverless would be unlikely to change that.
Though: https://www.bbc.co.uk/news/uk-england-london-19741282
Nicely sums up how the whole driveless tube/train aspect has been rumbling on for a while now.