it could be that the UK is mostly dense enough. there are a few very dense parts of the US where you've been able to get pretty much anything delivered (eg, NYC) for a long time.
I don't know much about grocery delivery, but I can think of two reasons offhand why it would be more viable. the first is that the delivery window for groceries is much more forgiving. if you have a cooler, you can keep refrigerated/frozen items at the correct temperature indefinitely. this allows you to load a lot more stuff in the vehicle and plan a more efficient route. the second is just a guess, but I'd bet the typical grocery order is larger than a typical food delivery order, so you make more money per stop.
I don't know much about grocery delivery, but I can think of two reasons offhand why it would be more viable. the first is that the delivery window for groceries is much more forgiving. if you have a cooler, you can keep refrigerated/frozen items at the correct temperature indefinitely. this allows you to load a lot more stuff in the vehicle and plan a more efficient route. the second is just a guess, but I'd bet the typical grocery order is larger than a typical food delivery order, so you make more money per stop.