On one hand, british standards say it and it may also be written on the box that tiles need mixing from different boxes..... On the other hand, they are white tile, there should be no variation in a single batch. Or at least not to that extent.
I know first hand the procedures W&F and other suppliers have in place so it is unlikely that you have received mixed batches, but human error can happen so although unlikely, not impossible. So thats why it is asked of whoever to do a final check before fixing.
Personally, i don't rate johnsons quality control that highly. I have seen many issues slip through the net and its not spotted until its too late.
If you still have the boxes, check the batch numbers (date to the right of the barcode). If they are all the same, johnsons need to get involved to do a site visit and if it is confirmed as a shade fault, get some quotes for re fixing, send them to the supplier and they can pay out and resupply new tiles when the factory agree to reimburse them.
If it is mixed batches, it will be more difficult as technically they shouldn't have been fixed. If spotted they could have been swapped over before installation. But you may still be able to get fresh tiles FOC but what else in terms of fixing costs will have to be negotiated between you, the fitter and the supplier.