And more on that eye colour

I'd also urge some caution re identification of the London bird as a pure Slaty-backed Gull, although I agree SBGU genes appear to be involved. We had a similar bird in California a few years back which we concluded, after much investigation, was too pale on the back to be acceptable as a pure Slaty-backed. During the investigation I looked at the Ujihara sites and concluded that there may have been a bit of circular reasoning there; i.e., it was unclear how they were separating hybrids from pure birds at that time (a couple of years ago). They had some individuals that were clearly mis-aged and 1-2 that I thought were mis-identified, which essened my confidence about the hybrid question. Perhaps it has improved since then.

couple of additional comments:

The eye seems too dark for a pure Slaty-backed, even if a subadult. Perhaps this has been discussed. I see no proof that the bird is in its 4th cycle although it certainly could be. Head streaking in basic plumage gradually lessens with age, on average, but the amount here is within range for "definitive" birds I believe, and some very old birds can show heavier streaking if their molt or hormonal cycles are off. Timing of molt also cannot be used to age gulls, especially vagrants. The best indication that this might be a younger bird is the eye color, but this seems too dark, even for a first or second cycle Slaty-backed (Peter Pyle)