I placed the bird on Facebook and an Icelandic birder commented that in his opinion it is a pure Glaucous Gull.
Yes, this one is imo pure glaucous. The primaries have all similar shading, apart from P7 and P10 which is whiter. This brown wash of the primaries is often much more variable (perhaps even the rule!) than for this individual. It does not appear to have darker tailband than the rump. Such brown wash over the primaries is also typical for many dark Iceland gulls.
Here's a bit of video of it (Don't forget to set the quality to HD)