I just joined this forum today and hope you find this reply in time...
There best way to do this is with a "change of address request" via Google Webmaster Tools. Here is a quick overview of the steps.
If you need more detail you can send me a message I'd be glad to help you.
Step 1: Register for Google Webmaster Tools (if you haven't done so)
Step 2: Once register you will have to verify that you own both sites. (This requires you to upload a google .html file on both sites to verify)
Step 3: After this is done select the "co.uk" site and under the Configurations tab (in the left nab) select "Change of Address"
Step 4: In the next screen there will be a drop down with you ".com" (this will not appear if you have not verified both sites), select and hit submit
Step 5: Your DONE
The great thing about this is method is google will then apply the
SEO and ranking from the .co.uk to the .com (which is what you want)
As a side note you will still have to setup 301 redirects as everyone has mention because if you have any linkbacks, banners or emails out in the "wild" with your .co.uk you don't want those to break.
HOPE THIS HELPS, CHEERS