Why not just freeze the whole loaf, like I always do? Then I just help myself to a frozen slice when I want it. I always toast mine, so frozen isn't a problem, but a slice will defrost very quickly especially after a short blast in the microwave. We have always frozen our bread this way. I don't remember not doing. Mr Blue still eats his awful white bread, and his loaves sit alongside mine in the freezer. None wasted at all.
Only Sainsburys in my town sell Burgen, and only the larger of the two sizes. However, they haven't had any for a few months now so I assume they have stopped stocking it. I did once manage to find the smaller size in a different town when we were on holiday. I am now on Hovis Lower Carbs, which is smaller, and has less carbs.