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Can anyone shed any light?
1) Did you have any prominent backlinks from .uk sites that are no longer?
2) Any idea if the bounce rate was higher for UK based visitors?
Also it'd be good to know how you're deducing what the UK SERPs are, I find [google.co.uk...] without cookies to be the "vanilla" results.
It could be a linked phrase issue where the algo [that would be the portion we call Penguin] has determined the singular is linked too heavily relative to the whole of the phrases in links to the page
Has it by chance been replaced by a different page on your site that might not even fit the query in a lower position somewhere in the top 40?