If this is how enforcement is taking place, and they are relying on the answer to be sufficiently accurate to charge based on it, then I think I'd ask them to show me the calibration certificate traceable to national standards, along with the uncertainties. If they can't produce that, they have no idea if the 10kg they've just measured should be 8kg or 12kg. Now the reality is that you'd probably have to pay, but at least then you've raised the objection if you later decide to contest it.

I'm also great fun at parties if anyone wants to book me. Baggage will be charged extra, though.