Same here :-)
The scales need a high level C-shaped hoop, say at 1.5m. That way the bulk of the weight of long yet light items is maintained downwards onto the scale, not "lost" by falling onto the framework... or distributing themselves all over the floor.
M&S and Asda machines work ok on light items like diet hot-choc (22g?) if you drop them into a bag, but not if you drop them onto a loose bag which cushions their impact (not registered).
The B&Q units appear more industrial. I wonder if their scales are capable of weighing heavier items at the expense of precision - such as every 50g instead of every 1g like supermarkets. That might not help discrimination of light objects.
The most laughable part of B&Q is "take your items ... ... ... <delay> ... ... do not forget your receipt". I can not help thinking it would be more logical to say "please wait for your receipt before taking your items".
Ah, usability... that post production & commissioning process :-)