Doesn't necessarily have to be their primary domain name - right, and I think it'd be worth it to an agency on '.com' to prevent a competitor (even an up-and-coming) potentially achieving a foothold over a globally very attractive market...
2) Also, the 'quality over quantity' argument? What is the reputation/specialisation in the .UK in terms of SEO? Additionally, could the breadth of '.com' and 'SEO-markets' it covers leave an SEO agency at risk/vulnerable to dilution?
3) .UK comes with trust but also accountability. Which might, in fact, be the strongest argument for the investment.
Saying that I wouldn't have put them up separately, but each to their own...