NotAnother try Iceland for frozen veg and basics. I think they do free delivery over £20. There are usually plenty of slots for my local. Otherwise I would start by looking at the bigger supermarkets (Tesco, Asda..Sainsburys..Morrisons) as they might have more flexibility to fulfil orders.
Our Deliveroo can pick up orders from the Coop and some of the "Local" Tescos (there is bound to be more choice in Central London).
Aldi have introduced "Click and Collect" round here and you get to see which shops have slots nearby fairly painlessly. I'm not sure how good they are at subs (i.e. if you ordered 2 pints of milk, would they give you 2x1pint if they were OOS).
What about all the restaurants that have suddenly had to close. Are they doing anything creative with all their stock?