Supplementary unit prices
Imperial unit prices are commonly shown as well as metric.
These are allowed provided they are not more prominent than the metric ones. However, UKMA thinks that they are potentially confusing, as you cannot compare the imperial price with the charge shown on a receipt.
Why different units are used
Where possible, unit prices are per kilogram if weighed
or per litre if by volume.
However, in some cases it is more consumer-friendly to use smaller units. For example the unit price for breakfast cereals is per 100 g because of the wide range of carton sizes on offer.
Likewise cooking oil has a unit price per 100 ml.
The unit price for herbs and spices is 10 g because of the small weights involved.
Full details are given in Schedule 1 of the Price Marking Order.
UK Metric Association 