The 320k figure is supply for ten years accounting for leaving land for industry, schools, etc. Read the document I linked to on land availability. So the comparable figure is a GLA estimate of 640k over twenty years and yours of 800k over twenty years.
You're absolutely right that the London Plan doesn't plan for enough home building, even on its own evidence base. It acknowledges that it falls short of a conservative figure just to meet need over ten years (349k), and is far short of the level needed to cope with demand and keep prices stable (that would need 447k over ten years, or a whopping 894k over twenty years).
Jon Neale's point is very interesting.. you're right that the actual levels of home building, and the planning targets, are lower than both need and demand. There are then different options to deliver on that, including bringing the density of suburban London up to a more sustainable level. Just imagine the capacity if you could bring the suburbs up from around 30u/ha to 80u/ha! It would also need much less infrastructure, and might be an easier sell than building on the green belt.