Without recognising single persons, it would be much harder. You would have to consider that a person twice as close to the camera takes up 2x as much room as someone 2x further away. Generally, it can also be very hard to see people on a crowded platform and you can almost double the amount of people on the platform and see as little of the platform after as you did before. Wifi will probably generate better customers in that case. Add in the fact that you are still processing visual data and that you will get a lot of media attention and Londoners will lose trust in your services and you have a definite no-go for your project.