Perhaps fewer women are willing to spend the 24+ months required to do the Knowledge, learning the city streets and routes of London via scooter, and then take the required tests/appearances to get their badge. As a general rule, the London "ply for hire" cab trade is unlike almost any other city - these individuals work for themselves, not a cab company/dispatch center, and they take pride in their accomplishment.

In NYC, Houston, Dallas, Phoenix, SFO, Chicago, DC...name the city - reliance on sat navs and expectance of a generous tip have become common place. At DFW, it took a cab driver more than 5 minutes to try and plot the route to my company's office via sat nav, not find it, and then ask us to get in another cab. But in London where, despite whatever street or bridge de jour is closed, a black cab driver can still get you from point A to B (and then C) efficiently and as quickly as London's onerous traffic will allow - when a quid or two is appreciated, why would someone be so stingy as to not provide.

There are always exceptions, but as a rule, the London black cab experience beats any other city hands down. So why not round up a pound or two?