[Paul]

I've used Sainsburys and Tescos, I think substitutions and service etc will depend on the local store that is providing the service for online delivery. Some stores will be good and the staff will try to get you what you want with as few changes as possible and then you will get the stores where they just try to complete the order with whatever THEY think is a good replacement.
Having said that I have found my local Tesco to be very bad for delivery time slots as they always seem to hit bad traffic or roadworks when they deliver to me and turn up late LOL