The key difference between the US and the UK here is that all the major supermarkets do their own delivery service. You don't have to rely on third parties to get things delivered.
Ocado have recently changed to use M&S at the backend
what does this mean? when was this? I still see a lot of Waitrose products at the moment.
The regular supermarkets don't have that luxury, your shopping order is beholden to whatever is left out in the store and the judgement of the picker. I've worked in supermarkets in the past and the pickers are horribly underpaid.
[1] https://www.ft.com/content/b985249c-1ca1-41a8-96b5-0adcc889d...
[0]: https://www.logisticsmanager.com/tesco-invests-in-urban-fulf...