Don't spend any more time worrying..
I was heavily involved in building a sport based website here in Australia. We had long discussions about calling it www.[oursite].com or www.[oursite].com.au or both. We went with both, but the one that got all the hits was the one we used in advertising (.com). The other one only redirected to the .com anyway.
This was a pre-thought to a new website, however you guys are well down the track, with a definite brand, which includes the .co.uk if you change it now, it will have little to no impact on getting new visitors, or making current visitor think they are on a more global site. contrarily your articles have that UK feel to them, hence the .co.uk fits with that. you've got international users because of that.
Anyway, long story short keep it as it is, and conserve your efforts for more important business decisions, like giving readers free stuff for long boring comments to articles.
Biting the hand that feeds IT
