Perhaps it would be helpful to hear how others also concluded this was a Herring Gull? I grew up surrounded by mainly Herring Gulls - I was always amazed by the variation - even after 30 years of looking at them, I would still be caught out sometimes!
I agree they constantly surprise you! When picking out YLG in a UK context I like to see them in all aspects: in flight from above and below, on the water, standing etc and hopefully in direct comparison with similar species (HG & LBBG). Obviously we don't have all of that here.
The lack of an obvious ear-covert smudge, the smaller bill, and apparent lack of any wing covert moult all seem to fit HeGu better, but I admit confidence is only 90% for me without seeing the full picture. Of course, Lou's agreement here on HeGu counts for a lot too!
Tail bands and primary windows can appear more/less obvious depending on light, angle and camera settings-hence my caution here.