I also hope for others to jump in, so I can learn, but: this Gull looks nearly exactly like the 2cy Yellow-legged Gulls I saw last week in SW-Germany.
-confident, strong build and shape with large, square head profile (but so variable in YLG and its the species where head shape is the most dependent on posture, more than in HG, CG or GBBG. Reason: dont know? Gull experts: please help!)
-extensive washed dark greater coverts that merges into barring there.
-extensive dark centers to back and scapulars
-the right quite uniform brown wing-coverts
-dark underwing
Caveat: I have learned from others, that Caspian x Herring Gulls can look similar, but this bird reminded me so good of a YLG, that I couldnt hesitate to comment. And of course hoping for more comments to confirm or correct. I am still in learning phase when it comes to large Gulls and others!