This is the first time I’ve seen a 4. winter Herring Gull with such a dark iris. But it can harldy be anything other than a Herring Gull of some sort. Smith’s tend to have darker eyes than our local argenteus, but this would even be too dark for a typical such.
And of course Caspian Gulls do have dark iris, but they do not have this extensive winter plumage – the spotting of the head and breast.
I think I’ll just write down in my notebook: Herring Gulls can show a dark iris in 4. winter.