Hitwise has published some interesting stats showing how fragmented and confused the UK search market was last month.
As you can see from the figures below, searchers are faced with a wide array of search engines and most are probably unaware of the different results they see by using google.com compared to google.co.uk, for example.
Any seachers from the UK who accidentally use Google.com or any of the non-UK focused engines such as search.yahoo.com are going to see almost totally useless results for the most competitive queries.
Try a search for "insurance" and you will see a list of US based companies - not what people in the UK typically want.
Personally I would prefer to see Google displaying the exact same content on Google.co.uk to Google.com if they detect a UK IP address.
Of course they should provide the option to show traditional Google.com results but this shouldn't be the default setting.