I think this is a skua and although it's probably impossible to be certain, there's a lot going for it for long-tailed skua.
I'm struggling to find any pics of Sooty gull with such a neat dark breast band and paler head, whereas this is quite often seen on long-tailed skua - see
http://wwwsapphirepelagics.blogspot.com/2018/07/1st-summer-long-tailed-skua.html
Doesn't sooty gull have dark underwings in all ages? The pale underwing looks better for a skua.
The jizz looks fine for LTS, with the deepest part of the chest level with the forewing, tapering into a very svelte rear end
The primary flashes look bang on for LTS - I'm seeing white in the shaft area restricted to the outer few primaries, and none anywhere else.