A fee increase to that level in other cities might have the opposite effect of reducing ridership and total revenues, against a largely fixed expense base, condemning the subway to a real "death subway".
The externalities of transport are massive, if a small number of people device to spend 15 minutes more in traffic to save the 8 pounds subway fare, the costs to society are simply astronomical because now everybody else has to spend those 15 minutes more too to get to where they need to go, even if they don't have the option of public transport for their route.