If you own the .co.uk but not the .com, you've not got a defensible brand in the first place.
Example.
No one can register Lexus, Mercedes, Porsche, or other recognised brand with any reputable domain extension, because if they did, it would get taken off them faster free shares of Google stock.
If you are instead talking about cheapbathroomcrap.co.uk and someone else has the cheapbathroomcrap.com, then your choosing to defend the real estate, because of the keyword rich nature of the domain, would make sense.
Don't think you're defending the brand name by registering .org.uk (I'd bet you're not even a non-profit organisation in the first place, otherwise this would be the first one registered and the last one dropped). If the brand is going to get assaulted, it could come from reviews sites or social media, or blog posts by reviewers, or any number of other methods.
Bottom line: If you don't have a brand name you can defend, like Porsche, and if the domain name in question is keyword rich, then hold it, put some content on it and use it.