I do not remember ever seeing price per 500g

Yep, every item I see is price per kg. I can't think of exceptions.

In the UK it is typically either per 100g or per kg. Or per litre or per unit. Depends on the category of product.

Its unit price as required by directive 98/6/EC.

A unit is how the product is customarily sold (singly) in the member state.

So for items typically sold as 500g in Germany, all items of that type in Germany would be labelled with 500g as the unit size for comparison. That same product may typically be sold in the UK as 1kg, so in the UK they would use 1kg.

Are we talking about stuff that can vary in size but isn't weighted? Like kiwi's or cucumbers have a fixed price each while apples or bananas have a kg price. But I don't remember seeing processed food without a kg price.

The only time I remember having difficulty comparing was different formats of toilet paper when our usual brand was out of stock... #rolls × #sheets / pack price was a bit to much for me to do in my head. Yeah, I'm not really complaining.

Same in Hungary. Everything has price / kg or litre shown next to the item’s price.

I read panpanna's post as saying that it's not common to have different units, not that there's no price per unit value next to labels.

It's at least law in Sweden since a while.