Almost every black cab ride I've ever had has been great - and they most certainly know how to get round London better than a newly minted uber driver with google maps. However if you've never had a black cab with the driver spouting racist/misogynist/anti immigration rhetoric they whole journey, then you haven't lived ;-)
A lot of taxi drivers grew up when London was part of England, before it became a "global city". It's unsurprising that they miss those days when it was still culturally part of England.