Out of interest, why would a sparks want to give you a discount for rewiring a small kitchen? If you were a regular customer and a sparks is looking after half a dozen rental properties for you too, or there is another bigger job coming up, then possibly. Otherwise no, there's no shortage of good work in London, no discounts on parking and no discounts on fuel, despite buying it from the same garage 2 or 3 times a week.
There's also a fine line between questioning a trade to reasonably understand something as a customer/consumer/end user may find useful, and questioning to the point of doubting a professional tradesman and becoming a bloody nuisance in the process.
You mentioned early on you cut code. I'm sure if one of your customers kept debugging your C++ files and questioning aspects of how and why you've coded in a particular way, you'd quickly get arsey about it.
Treat the trade as you like to be treated yourself, then you'll find 99% of people you meet will be genuine, helpful and go the extra mile to ensure you're satisfied with their work.