Comparable sub-adult 'SLATY-BACKED GULL' from California

Here's a 3rd-cycle (by brown greater coverts and bill) SBGU accepted in California that appears darker backed and certainly paler eyed than the London bird

http://fog.ccsf.cc.ca.us/~jmorlan/sbgu012105.htm

It strikes me that the head streaking of the London bird may also be a sign of hybridism for a 4th+ cycle bird, that of vegae being darker, age for age, than SBGU. I also agree with you (contra Lethaby) that the back color seems to vary a lot in these gulls, whatever they are, at least according to what the Japan sites identify as Slaty-backed. In the end I still think the biggest issue is that nobody seems to know how to define a pure Slaty-backed Gull (Peter Pyle)