Looks like a 1st summer Common Tern. Too dark for Roseate, and too long-legged for Arctic.
First Summer (or 1st s Arctics) would be fairly unusual in UK (they tend to stay on their winter grounds for the first summer) also, the outer primaries don’t look dark to me so I would be interested how you (and others) are so confident in aging this bird? Terns are notoriously difficult to age accurately in non-breeding plumage as anything other than immature or adult, even in the field.
1st & 2nd summer Arctics are not unusual in the UK. Go to any Arctic Tern colony and you will see a few. Ok, ageing can be a bit dodgy, but 2nd summer s are usually more advanced than this bird - fainter carpal bar and reddish legs & bill. In any case this is not an Arctic as it is too long-legged, but the same things apply to Commons.
The article you referenced is June 2001, not 2011.
I can only see the bill as all-dark.
I was a little puzzled by your question at first, but when I looked again at the montage of the two terns I uploaded yesterday, it suddenly made sense; the annotation in the middle of the montage, referring to the dark and apparently worn outer primaries alludes to the Common Tern in the lower half of the montage, rather than Arctic at the top!
...Incidentally, I have noticed that quite a few 2 cy-type Arctic Terns (which invariably have a full, clean set of quite fresh looking primaries in June) start to moult the innermost primaries in late July, a moult that I have not found reference to anywhere? I will let you know if I make any progess with regard to establishing useful differences between 2cy and 3cy Arctic Terns.