And in that environment it is not viable. Or, not viable to SF standards, in that it cannot sit on their arse and print money while their devs churn out fancy looking apps. They're competing with the local taxi company with a dozen cars and a phone number.
Let's not forget that in that world uber drivers are unemployed while taxi drivers make more money.
It's not like Uber is the only app like this in London. I have at least four different ones on my app, and Uber is rarely the cheapest as it is.
Uber has captured market share by being visible, and covering the whole city so you don't need to care which companies cover where you are. There are a few others that covers all of London and aim for the same over-supply of drivers, and most of them are not cheaper.
Overly simplified solution: Raise prices to cover a decent wage.
To be clear, if Uber are unprofitable, that's a business model problem, not a worker problem.