Getting around in London by taxi

Why is it that taxi drivers consider themselves to be professional? I assume from their use of the term that we can define 'professional' as one of the following:

Sloppy
Inconsiderate
Rude
Aggressive
Dangerous
A law unto themselves....

I realise that taxi drivers' earning are dependent upon them making the best use of their time, but that does not excuse some of the appaling driving standards displayed by some of them. It would be interesting to put a few taxi drivers through the driving test again. It might well be a bit of an eye opener. Whenever I see a taxi, my immediate though is to keep well clear. If it is behind me, chances are it will be driving far too close, so I will let it go. There is no point making any kind of issue out of it. (I've tried and all I got was an earful of abuse).
If you, as a taxi driver, find this opinion offensive, ask yourself about your own driving standards. Get the Highway Code out and see where you fit in. You might just be surprised.