I think I have a solution to my problem, just in case anyone has the same specific issue:
Tesco allow you to check out initially and then edit the order but leave it un-checked out. You can then open the same order on a different device and add items - i.e. other family members can add to the open order. There doesn’t seem to be a time limit to check out on editing the order, other than about midnight on the evening before delivery.
Once finalised, e.g. the night before delivery is due, I then check out, complete the SCA and hey presto!
The only time it needs checking out before this is if we need to book another slot or change something in another order.
The one variable is what happens if we forget to check out? I’m going to experiment by checking out and then going back in, adding some extra beers and not checking out - if I get the beer, all is well!
I wonder if this works with other supermarkets too?