To cut a long story short - builders have put up the shell for me on my
extension, the floor in the kitchen is concrete 'screed' and is not
level.

This only came to light after the kitchen was fitted and the gap
between the cabinets and the floor showed up as an inch larger at one
end of a 12' run than at the other...

The builders have seen this and agreed it's not level (although their
solution was "Your fitter should have compensated for it, sir").

Re-laying the floor would mean stripping out the cabinets and flooring
that has already been fitted, which I'm not really interested in doing
due to several months of upheaval for my family already while the work
has been carried out in the first place.

On a 24K build I have a 1K 'completion fee' (I know, I know...), but my
question is if 1K is the price of a successful completion, what sort of
discount should I expect when the floor they've laid ain't level and
leaves the kitchen finish sub-standard (i.e. big gap between plinth and
cabinet).

If you saw my other post regarding water getting under the laminate
flooring, then yes, it's the same builders.

Cheers

Rob