Are you looking at results for google.co.uk, google.com, google.whatever?
You can select the google of choice to view the results if I recall correctly.
You need to check on the exact same site, and also the same way, did they use quotes or not? Did they select view pages from uk or not?
Plus - the results in webmaster tools are delayed, so there it is not certain that the serps will be the same now as when the data shown in the report was valid.
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Aggh, sorry I thought you were in webmaster tools - forgot you said analytics...
There may be one in analytics, not sure of the top of my head.
But if you look in webmaster tools for your site (under statistics/top search queries) you can select the search engine the user used, and the time period they were valid.
You can filter these down to specific search engines (ie Google, MSN, Yahoo) it may be that you are ranking for these search engines and not for others.
Procedure:
Login to Analytics -> Your Domain -> Traffic Sources -> Keywords -> Click your keyword to break down -> then click on "segment" drop down box, select "source" to show which search engine referred which users