> Ocado can only work with packed goods - not weighed vegetables for example

Not quite. Packed yes, but for many vegetables they have both item count and weight-based packages, e.g. "4 potatoes" vs "1kg potatoes".

I think that strikes the right balance.

Interesting, however in my case we have someone who eats a banana each day with breakfast. There's no way to buy 7 bananas in your weekly shop!