You'll also find that a significant number of minicab drivers aren't white men, and those who are are often from outside the UK.
Being a black cab driver is a career move. Many people who decide to do it move from another career, particularly where that career ends early (like firemen, for example). A lot of time, money and effort needs to be invested to become a London cabbie.
In contrast, minicab drivers often find themselves doing it because they need a job to feed their families. I tend to chat with them and have discovered that quite a lot of them do consider the move to getting a full-blown licence. Some do, as the extra money and job flexibility is worth it for them. Others don't, as they don't see it as being worth the effort.

All I can say for women is that while some of the men I know would consider it (one actually has done it) none of the women would.

As for tipping, I tend to round up a bit. If the driver has made an effort or has been chatty then I might add a few more quid. I tend to avoid black cabs as a rule though. Most of them are fine, but I've had one or two being really offensive and I prefer not to take that risk.