You can use both. But not with the same content or not without a 301 redirect of some sort. If you are competing locally at first, then redirect the .com to the .co.uk site and do all the local SEO stuff on the .co.uk site to compete well. As the company grows, then the .co.uk site can be more company related and the .com can be product related. In fact, you can do that now if you want. You do not have to park either site.
Now if you decide that the .co.uk site should be a company site, then you can cover your products but refer your users to full details on the .com site and vice versa.
I think it all depends upon how much content you can create for both the company and the product. If you can create enough content that each site can stand and compete on it's own, then okay. Otherwise, you may be better off creating the .co.uk site in full and redirecting the .com site to the .co.uk site. If you redirect the entire site with a 301 redirect, you should be okay.