I think if they default it to only search UK pages it would be bad for traffic on my .com sites, but I guess the .co.uk's would get more traffic - if that were to be the case people will just end up building virtually duplicate sites using the different country extensions.
no country redirect
is that something that needs to be added to the G cookie - I can't see it in the preferences.
If you are a UK site with only a .com domain and not .co.uk you could be screwed. As on Yahoo.co.uk (from personal experience).
I think that this shouldn't be the case on Google. If you are a UK site and you only have a .com domain, got to ensure that the domain is with a UK server and not hosted elsewhere therefore ensuring you shouldn't have any difficulties.