you should have no problem ranking a .com, in fact if anything it should be easier!
I rank all my sites in .co.uk (the web) and .co.uk (pages from UK) and Google US without having to change the target audience in webmaster tools. The secret is where the actual website is hosted. If you want to appear in Uk search engines then get a UK based hosting provider. All my sites appear in UK and US search engines without touching webmasters all by just having a UK host. Be carefull though i have been scammed by Indian hosting companys claiming to be in UK.