So minicab companies pay more and charge less than uber? How does that work economically? And wouldn't everyone just switch to the mincab companies, both drivers and passengers, if that's the case?

Varies very much by company. Many of them are small operators covering much smaller areas than Uber. Many of them will intentionally keep far fewer drivers available to keep their drivers busier. I'm sure there are plenty who are much worse than Uber too, and there's plenty of room to get rid of more of these operators.

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.

Is it impossible to operate a mini-cab business more efficiently than Uber to a similar standard? I suspect it's pretty hard NOT to.

And wouldn't everyone just switch to the mincab companies, both drivers and passengers, if that's the case?