Same website with different extension (com and co.uk)
Many people use both domains but if you target UK market, i like to have .co.uk as main domain...have you got any web site in which you use both extensions ?
Google has simplified the process by providing an option that can enable you set your targeted country from your webmasters tools account so even if your domain is not targetting your required geographical audience, you can be able to correct the error from your WT acct.
It depends on the business you choose to do and the geographic area you are targeting, if you are targeting uk then go with co.uk extensions.But if you targeting other countries, it is best to go by with .com extension.