There were four white-winged gulls at the Marina in Ashbridges park this afternoon, two adult and two sub-adult individuals. I would have taken them all for Glaucous gulls, due to their nearly Herring gull sized bodies. However, a couple of things were puzzling me.
1. They weren't bulky-heavy looking in flight, yet seemed bigger than Iceland gulls I have experienced in the past.
2. The two adults both displayed gray wingtips, not all-white, as expected for Glaucous.
3. The sub-adult individuals had mostly short black bills with green-pinkish base, quite different from the normally distinct black-tipped long bills of you Glaucous gulls.
Two crops are shown below (these are hand-held digiscoped images)