Hi-Lo bread is (unfortunately) a very low volume product. Sainsbury's systems have difficulty coping with it. When availability at my local store became very erratic I had a long discussion with the Bakery Manager. They sell on average 6 loaves a week in a fairly large store. It is delivered with the cakes (!) in a small box containing 4 loaves. The automatic ordering system can spread out deliveries anything between 4 and 10 days, which may well not match very well with the pattern of when people who want to buy it go into the shop, sometimes meaning they are left with one or two after the sell-by date, even if there are also frustrated people on other days in the same week who can't buy it because it is out of stock then.
What I have done is buy it whenever I am in the shop, and if I go short order it from Ocado. The latter option I guess might now be the only one for you.