Google.com is universal you can used it anytime anywhere.. but if you are focus to get traffic from a certain country.. you should used SE on the specific country you are targeting..
You can specify your country in Googles webmaster tools, if you have a .uk domain I think google automatically puts you in the UK Results. But you still get traffic from all over the world
lol I'm not sure whether anyone else has chimed in about this, but hey...have you tried Google.co.uk ?