You raise several interesting points.

On the matter of plagiarism in general: Yes, you're right; if a Wikipedia page is remarkably similar to a page elsewhere, that doesn't mean that the WP page plagiarized the other: it could have been the other way around, or something more complex may have happened.

On this page: Its history shows an almost uninterrupted growth to this state, before the trimming started. I haven't examined the growth, but yes, gradual growth (as opposed to sudden jumps in size) suggests that the editorial work went on here. So perhaps I was too hasty.

Here is Graham's page about himself. I now notice that it says at its foot Excerpt from Gallery Press Release, 2008. So it's possible that it copied the press release which itself was a copy of the WP article.

So maybe I have an "overly critical attitude". But if so, it's an overly critical attitude toward the editing, not Graham's work. (I've seen some of his early work from Beyond Caring and think it's very good; I know little about his other work.)

Why the apparent concentration on Japan? First, it's hardly a concentration; it's simply the addition of a sourced fact. And, if you're interested, here's how it came about. Having created Category:Photography in Japan, I populated it. Racking my brain for non-Japanese photographers who'd done a significant amount of work in Japan was a pleasant activity for a commuter trip or three; I also emailed a couple of friends more knowledgable about the matter than I am. Graham's name came up, either to one of them or to me. I was certain that he'd done work in Japan so I put this in  in accordance with WP policy, backing up the assertion with a reliable source. 

This was a contribution of time and energy, though admittedly a tiny one. Graham's work seems to have many admirers. I was hoping and expecting that among themselves they'd rebuild the article. I'd be willing to help with this. While not working on Graham's article, I've been working on articles about other photographers.

So let's turn back the clock to September 2008, before the cuts. Here's a paragraph:

Graham was the first photographer to combine the sensibility of contemporary colour photography with classic British social documentary. In 1981/2 he made A1—The Great North Road, a book of 40 colour photographs taken along the length of the British A1 road, which had a transformative effect on the black and white tradition that had dominated British art photography till that point. This work, along with Graham's later photographs of the 1980s -  the colour images of unemployment offices in Beyond Caring (1984-85), and the sectarian marked landscape of Northern Ireland Troubled Land (1984-86) - were pivotal in reinvigorating and expanding this area of photography, by both broadening it's visual language, and questioning our notions of what such photography could say, be, or look like. Photographers like Martin Parr made the switch to colour soon after, and a new school of British Photography evolved with the subsequent colour work of Richard Billingham, Tom Wood, Paul Seawright, Anna Fox, Nick Waplington, etc.

Let's suppose for a minute that this is legitimate WP text, and not ripped off from somewhere else. Its content is a mixture of the easily verifiable and the arguable. Was he the first photographer to make this combination? Did this one book have a transformative effect? Why "British art photography" (my emphasis)? How were they "pivotal"? This stuff would have to be sourced. (Yes, writing for WP is a bitch.)

Backing for some and perhaps even all of this is probably within Williams' text for How We Are and Mellor's for No Such Thing as Society. Do you have either of these?