I think this is a leucistic Herring Gull? I can't find any other explanation for how pale this bird is compared with other 1st year Herrings. The possibility that it is argentatus has been raised however I would expect that to be bigger and bulkier. I also don't think any hybrid explains its plumage.
Funny how the wings look much paler when closed!
Would be very interested to hear others' opinions.
Thanks in advance.