With the new photos, I am moving my vote from undecided to reasonably definitive Glaucous.
Jizz is helpful, but just doesn't hack it when it comes to large white-headed gulls. Plumage is complicated because we have a first winter gull with worn and bleached features beginning to moult into first summer, but I don't see any reason to suspect aberrant Herring Gull. (By the way, the bird in the linked photo from Recovering Scot looks like it has longer primaries, shorter neck, and a less cleanly demarcated bill with more black, i.e. a genuine Iceland, but that is just my view.)
What I see is confirmation of structural characteristics for Glaucous, a bird that can have virtually identical plumage to Iceland, and can supposedly only be reliably distinguished on structure, bill and such. The most obvious confirmation is that the primaries genuinely are relatively short, which is a very strong indicator for Glaucous. The bill looks fairly definitive for Glaucous as well, with the wide pink base and the short, cleanly demarcated black tip.
For those of you who have Olsen and Larsson, in the description of plate 247, he takes a juvenile/1st winter Iceland with a bill that is particularly glaucous like, and invites comparison with the genuine juvenile/1st winter Glaucous bill in plate 220. To my eye, the bill in the subject bird bill is virtually identical to the Glaucous in plate 220, and distinctly different from even an example of range of variation in an Iceland.
Fun stuff...