Sgt_Kickaxe - 4:11 am on Feb 15, 2012 (gmt 0)

No on-page tags are needed, you can sign up for and log into your Google Webmaster Tools account and assign a geographic location to each domain. Obviously you would assign a U.K. geo-location to the .com.uk site and I would simply leave the .com without a geo-location set (perhaps at most set up some code on the .com to redirect to .com.uk if it detects a UK visitor?)

I can't guarantee Google will adhere to this, they don't trust webmaster provided information very often, but it's designed for this.