What a strange world he lives in. I don't think it is the same as ours. Or, if it is, then he's clearly got some serious limitations in the thinking department if he cannot imagine a situation where a home owner builds an extension by contracting in individual tradesmen who have no relationship with each other, or by a mix of that and some DIY.Really ? Go read the regs again - carefully. While the majority will have a principal contractor, I can think of some domestic projects where there would be no single contractor fitting the description.There will always be a main contractor, he is called the principle contractor.
There is always a principle contractor, whether there are single or multiple contractors. Always.
I expect that this site is full of such examples - odd that he's never seen them. Maybe he just refuses to accept that people are telling the truth.