Not that it was completely without problems, but compared to say NYC cabs they were worlds apart. Sure, there was a problem for a while with rogue unregistered cabs, though IIRC that was mainly minicabs and relied on intercepting despatch radio messages, but there were some black cabs. The cliche of not going south of the river held up to some scrutiny too. Uber of course go with phone you then just don't show if they don't like the route and waste half and hour of yours. At least it was a two way conversation with a cabbie.
That London hasn't put a blanket ban on diesel cabs in the low emission zone isn't really the cab's fault - that's firmly on the authorities...