It could just as well be a BSA or a Triumph. Scores of bikes of that era used those lugs. I think there are a couple of things that will help you identify it.
-It's wearing 1930s fittings (like the mudguard fittings) and it has a head clip. Head clips were all but obsolete by the 40s. I think this is a good enough indicator that it is a late 30s frame. The angles are not that slack, so I think it's not early 30s. I see what looks like chrome on the fork tips and stays. This tells us that it's probably not a wartime frame because chrome was scarce then and less common on bike frames. You tend to see it pre-war and then some time after, but not during war years.
-You say it's Reynolds 531 - how have you identified that? If it is, then it will narrow down the builders. for that era, the seatpost should be 26.8mm for Reynolds 531. 27mm and 27.2mm was later Reynolds 531.
-If it is 531, it doesn't have modified 'fancy' lugs. A lot of hand built bikes (lightweights) had modified lugs, while probably all factory built bikes had standard lugs.
-The tops of the stays were another place where the makers were able to distinguish their bikes from the competition. If you can spot which builder did that to their their seat stays then it might help.
I think it's a factory built bike. Almost certainly a BSA, Sunbeam, Triumph, Sun etc if it is. It could well be that Sunbeam since it's got the Cyclo gear hanger.