Why would it matter? You need so few subway drivers/conductors that does it really matter if you need one per train? Taxis are probably average a driver per 1.5 ish customers. Subway/train is hundreds. This reminds me of optimisation of non critical code. Sure its nice, but the cost/benefit is just not there.
It's political. In the uk, train drivers unions are one of the last meaningful unions left and the right wing government hates that. Also, London has a left wing mayor and a very good public transport system. The right wing government hate both the mayor (he's heir apparent to lead the party nationally) and the concept of public transport. That's also the real reason private train operators were bailed out but public…
I'd love to know by what scale you consider the British government "right-wing"?