I'm just basing it on our experience in buying scales for weighing people at the surgery. To get accurate scales for weighing people costs a fortune, because the engineering needed to get precise measurements needs very fine tolerances, also they have to be calibrated regularly to stay accurate – which in most cases they're not. Similarly calibration weights are also very expensive, and have to be very carefully looked after.
In most cases in GPs surgeries and hospitals, the scales used aren't actually that accurate, because maintaining accurate scales costs a lot of money, and precise measurement isn't needed.
For those reasons, I find it very hard to believe that a set of scales costing £60 are likely to be particularly accurate.