Welcome to Flyertalk and welcome to the BA forum ABZ.

ALG is a special route since it comprises of two groups of travellers who wouldn't make up much of the rest of LGW's customer profile. The UK's Algerian diaspora, which has long roots in London, and oil / gas workers, and I'm guessing you are in the latter group. The first group typically make one annual trip to see their family, with huge luggage requirements and they book this way in advance, often as soon as the flights are put on sale. Though it's not that far away - Algiers is about the same distance as Naples - the idea of quick trips home doesn't seem to work, presumably due to the cost. So by the time the oil workers pitch up, on much shorter timeframes, the seats are gone. I vaguely recall BA also has staffing, weight and cargo limitations on this route so it wouldn't surprise me if BA doesn't actually sell every available seat on the route.

So it may be KLM or AF for you here.