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This has only been happening since this weekend - USA sites are dropped down the rankings and UK sites are bumped to the top.
To be included in "Pages from the UK" results you need
1) a .uk domain
2) a UK IP address
A UK postal address in your Whois does not get you into "Pages from the UK" results.
On most searches there is nothing different other than the usual minor differences between serps from different datacenters. Most users will therefore not notice the difference.
On some searches the serps are dramatically different. (Sticky me if you want an example.)
For example in one search in google.co.uk, UK pages have been boosted to #1, #2, #3, #5 and #8 from positions in the google.com serps ranging from #8 to #60. Other pages have been pushed down the serps accordingly, but retain the same relative rankings to each other. Some UK pages are not boosted at all, but some others lower down the serps are also boosted to some extent.
i.e. Some, but not all, UK sites are boosted in the serps.
Unlike the "UK Pages" filter criteria, a .uk domain or a UK IP address are not enough on their own to deliver the boost.
Also, other UK pages are recognised and boosted without a .uk domain or UK IP address.
None of the following factors on their own deliver the boost:
UK IP address
UK on page content
UK Whois info
UK regional DMOZ entry
i.e. there are examples of sites with each of these that don't get any boost.
Most "UK" .com sites that get the boost have high PR, often 7, 8, or 9, but there are exceptions.
Many "UK" .com sites that get the boost have UK regional DMOZ entries, but there are exceptions.
.gov.uk and .ac.uk seem to do well, but there are exceptions. This could be because Google recognises that these domains are an authoritative UK indication, or because of a high number or high proportion of UK backlinks.
Because of all the exceptions, I see some of the possibilities as:
Some specific pattern to backlinks
An overall "UK" score based on a number of factors
A "UK" score calculated periodically, with some sites not yet analysed.