They are most certainly roundels, you can just make out a dark halo around the white spots.

Exercise markings usually took the form of painting the underside of one of the wings black or various designs of stripes.

Gas patches. The most familiar are the yellow squares or diamonds applied early in the war. They could either be painted on or they came as a square of material that was stuck down with red doped tape along the edges. Not usually found in pairs. Circular, white gas patches appeared later in the war. They were applied to the noses of Lancasters in 1 Group, perhaps because they were trained in gas warfare. I have also seen a photograph of a Corsair with one, otherwise they are very rarely seen apart from Lancasters.