What would be the effect on the search-engine rankings for a given keyword or keyphrase if one declares the document language in the head as either British-English (en-gb) or American-English (en-us) or another dialect group, instead of using a non-specified dialect of English (en) as the document language? Would a page lose its rankings for British-English if the document language is en-us? How important are other factors, such as the IP location of the person who performs the search, the location of the server of the website and the language of the backlinks to the targeted page?
I think you've made this alot more coomplicated than it actually is, but i don't think it really matters. i've never heard otherwise, but i think if you put en-gb, google will assume you are based in the uk and you will be ranked in google uk and when somebody searches for uk pages only. But it's just a suggestion, don't take my word for it.