How do I bathroom / kitchen / extension? etc.

Didn't quite work like that and my dad's got an account. They basically said what do you want I said I have this, they designed it and at the end said the price was X or Y if I built it myself.

There really wasn't much of a debate, the days of trademans saving fortunes over the customer price is gone from experience.

Fitting it though its fine apart from the cockups Howdens have made and just like most other kitchens I work in that are 10's of thousands.

it seems like Howdens' is set up to allow builders to choose how much they can rip off customers by

Yep exactly that. But in their defence if you do the admittedly annoying negotiation dance they can be very good for the money.

I do hope your answer to his initial question was 'as little as possible'.

I get discounts from my suppliers as I have been loyal to them for a long time (and therefore give them a small but reliable amount of sales).
I do not pass that savings on to my customers, i charge them the standard rate for my supplies.
I take that as profit and therefore pay slightly more tax on it.
I am an intermediary in the supply chain and want to get a slight payment for my time and effort.
It has taken time, money and energy as a skilled professional to get to the point where i am paid for my work, and for the years graft where i did not.
I do not go into Sainsbury's and demand that i pay them the same for a tin of beans as they pay their suppliers, and I expect that most DIY heroes don't either, although i would enjoy seeing them try.
I'm also aware that many tradesmen will use this mechanism to unjustifiably add cash to their mattress; but the practice is not set up to "rip off customers".
It's how you run a business and earn a living, and pay the wages of those who work for you.