Google.com is an entirely different beast from Google.co.uk -- and webmasters have taken the time to discuss some interesting observations on the .co.uk end.
Some are seeing results jumping around with major reshuffling. On a related note, some are seeing the same but are getting results only for very long tail searches.
Another observes issues with geotargeting. The specific scenario:
For instance many phrases for a 14 year old UK hosted global authority .com is #1 on G.com and is halfway down G.co.uk and some phrases not even ranked at all.
Forum members also observe and believe that Google.co.uk is using click-through data. That's interesting. I guess people know how to rank better, then!
Forum discussion continues at WebmasterWorld.