Whys that then? I thought that I can run the cable and get an electrician to fit the consumer unit?
No - you can't.
The electrician has to certify that he did all of the work - the design (which includes sizing the circuits, selecting the cable routes, deciding on methods of installation), the installation itself, and the testing.
He has to certify that
he did it, that
he exercised reasonable skill and care in doing it, and that to the best of
his knowledge and belief it complies with the Wiring Regulations.
He has to certify that it complies with the Building Regulations.
He cannot and will not do those things if you present him with cables you've installed, a CU you've bought, and a request to just connect it up.
It's not just "job assurance", it's regulations and laws.
Also the prices online for the consumer unit complete with MCB's are extremely competative to those available to a couple of electricians I have spoken to so I thought it made sense to buy it?
If they are cheaper then it's because he is adding a markup to them.
So, let's say that the total price for the rewire, with the electrician supplying the parts, is £4,000.
And let's say that that is made up of:
Materials £750 (cost to him)
Markup £500
Labour £2750
So - you see his quote:
Materials £1250
Labour £2750
And (remembering that you won't be able to buy as cheaply as him) you think "Hang on - I can buy the materials myself for £900 - that's going to save me nearly 10%".
Now - one assumes that the electrician has struck the right balance with his £4K price and his gross takings of £3250 between being competitive and being able to put food on his table.
So - faced with getting £500 less, will he:
a) Say "OK", and not be able to pay for his shopping for a few weeks?
or
b) Just add it into his labour costs, at which point your rewire price becomes £4,150?