If you search at Google UK for keywords such as [search engine optimisation] or [personalised napkins] or the like, you will get US based spelling suggestions from Google.
Here are some screen captures:
The old S vs Z in US vs UK English.
As you can imagine, this is not just annoying for UK searchers but also can be a bit insulting. So far, Google has not commented in any of the threads I have seen report this.
Forum discussion at WebmasterWorld, DigitalPoint Forums & Google Webmaster Help.
Update: Google seemed to quietly fix this in the past 24 hours.
Update 2: Here is a statement from Google over 24 hours later:
We recently introduced a change to the spell correction feature on the google.co.uk domain. This change introduced a bug where we were suggesting American English spelling refinements. We have temporarily rolled back the change while we fix the problem.



Comments:
No Name
01/14/2010 01:20 pm
Dreist. They try to force their spelling into the brains of young british googlers.
Darrin Ward
01/14/2010 08:08 pm
Interesting topic. I'm originally from that side of the pond and until I moved to the US I would spell with the s instead of the z. If I were still there, though, I don't think I would be insulted. Well, maybe initially. But the US output of content published online must be far greater than that of the UK, based on population and Internet penetration numbers. So, if the spelling modifications would yield more/better search results, then I think the spelling recommendations are valid. I guess the searchers don't have to use them.