Hi,
The difference between results from Google UK and Pages from UK is this: Google UK - The Web will display all the sites that rank in the UK no matter where the site originates from. For example - If I start a website and host it in South Africa but I receive a whole bunch of links from the UK for a term, it will be possible that my site will rank in "Google UK - The Web" for that term.
However, "Pages from UK" will only display sites that originate from the UK. So my site from South Africa wouldn't appear on these search results no matter what I do (unless I try hosing the site in the UK and I don't have country specific URL such as ".co.za".
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I'm not 100% sure but I do believe that SEOMoz will track results from "The Web" and not "Pages from the UK".
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I don't know of any statistics about this but my guess would be that there aren't many people choosing "Pages from the UK" compared to "The Web". (Please try confirm this though)
I hope this helps!
Shaun