4000 drivers at an average of around £70K per year is not chump change (£280M) + employer costs for staff. Out of a budget of maybe £5B per year, it is not insignificant but I would expect there are other areas where savings could more easily be gained.

Maintenance is crazy due to the short maintenance hours and therefore everyone getting paid enhanced rates to work 0:00 to 04:00. Significantly modernizing signalling and control would be expensive but would probably back back over time.

A few years back I was in a control room in a tube station and saw some steampunk looking machine that we assumed was related to the old destination boards on the station, which were long gone but the machine was still operational and left alone. Why? Because who would risk breaking something just to remove some random old piece of kit!