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.
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.