I use Sainsbury's [choice includes Asda & Tesco, who frequently have zero slots in our area - I still see their vans, just not as often]. We have a local branch but the deliveries come from a warehouse about 55 miles away.
Cost of booking a timed slot varies, but we've been happy to take the cheaper times [ie late evening / night].
Very few substitutions / product unavailable, or damaged/spoiled stuff over the past few months that I can recall. And that includes the few frozen items we buy. Those that are "below par" have been refunded by the following day.
The funniest substitution - we get a weekly rag "RAIL" and once recently we got a TV weekly guide instead.
Pleasantly surprised by how often the truck has turned up early - the drivers will ring to see if you are available, best on that was over two hours before the booked time. I am usually WFH so not a problem, only once were we out of the house / area at the time of the call, and we got back about 20 minutes before the start of the booking, and the driver was only a few minutes away at that point. [I reckoned he was parked at the farm opposite, several times in the past year the van has been there either before or after dropping our stuff.]
However, the last trip was not so timely, for the first time. Several hours before it was due we were told it would be late and then we were asked to re-book, for at least 24hrs later. Turns out the petrol panic & a little bit of suspected covid in the driving teams had messed up the routes.
Can't get ocado - outside of their area, although I have seen their vans once or twice. Oh, Iceland and Morrisons also "do" around here, so we do have some choice, but as we are satisfied with them, we've stayed with Sainsbury's. I have heard various [horror] tales about the others as I know a few people who were [or are still] isolating or don't drive.