Larus argentatus is from Northern Europe, Scandinavia, Finland, Baltic Sea and Germany which is generally larger and heavier than it's subspecies argenteus of Western Europe and Great Britain.
Variation in plumage from light to dark birds, then add the intergrades where subspecies overlap making identification impossible ..
The following three birds to my in-experienced eyes are good candidates for argentatus, all being more distinctive in flight with the paler coverts & mantle contrasting against the darker secondaries. Pale window in primaries with the outer web showing pale cigar shapes. The tail band is dark brown with pale edges, barring & spotting on upper-tail and rump against a white base colour.
Pale head, darker masking around eye and a stout bill.
Reduced pale barring on outer greater coverts.
Underwing coverts dark against the paler flight feathers.
Tertials variably oak leafed with dark centers.
To me they look good for argentatus, maybe i'm wrong if so contact me ..
Bird # 1
Bird # 2
Bird # 3
Three very distinct birds compared to the normal brown look that dominates most of the Herring Gulls ..