Is it not that they don’t have any slots for your area for the foreseeable future but that does not necessarily apply to all areas? I imagine it is down to the availability of vans and drivers.
Browsing the Ocado website on Sunday I found half a dozen slots for the next day for the area of S. Londob where my sister in law lives. She is self -isolating and (foolishly IMO) thought she could get by on her own, instead of going to live with her sister who is a Dr.
Anyway, I grabbed a slot on her behalf and the two of us put together an order which should see her comfortably into next week. I have one booked already for here but not until 4 April and no sign of any more at all for my area thereafter.