If you're working for business publication, it would be considered "good practice" to have someone other than the author, or in this case the translator, to do a proof reading just to catch the typos and other "little things" that creep in. In your case, finding a proofer who's experienced with UK English would be the obvious way of getting a good UK-English text.

Many very good "proofers" work freelance, and you can work via email with them. Finding a good one may take a bit of work, but the benefit to be had can be well worth it. In your case you'd have the little extra of finding a "Brit" proofer, but it shouldn't really be too difficult. A quick Google for "proofreader" might indicate whether there's a handy covey of them somewhere in your region. You might find someone via a search for "Temporary Employment" agencies, but some of these have a tendency to "promise everything" regardless of the skills or lack thereof of the people they offer.

Many of the major publishing companies keep a roster of freelancers, and usually are willing to share recommendations, since it's to their own advantage to keep their best ones occupied (and happy) when they don't have work of their own for them.

In my own area, I'd probably just drop in at the overpriced "atmosphere" pub where all the green card Brits hang out and ask if one has a wife (with her own greencard?) who'd be interested, ... (in proofreading) ... maybe.

Using a separate proofer in no way denigrates the author. It's a mark of professional work that your business should be willing, if not eager, to use.

John