Editorial agencies are not without their problems: there are plenty which offer very expensive bad advice; and there are a few which are out-and-out scams (Edit Ink, anyone?). Some, though, provide a useful service: Sharon Maas
Writing a good book and using a good editorial agency to improve it won’t guarantee immediate publication, though, as Sally Zigmond discovered; and even the good agencies won’t always get it right—if you write in a genre they aren’t familiar with, or in a style they don’t particularly like (regardless of its literary merit), then you’re not going to get the best advice. And using the wrong agency for your particular book, even if it’s a good one, can be an expensive mistake to make: those reports can cost hundreds, if not thousands, of pounds.
If you’re determined to use an editorial agency to improve your book, how should you proceed? You need to find a few reputable agencies: either through personal recommendation, or through your own research. They must be run and staffed by people who are qualified to do the work: either by having appropriate editorial experience, or by being published writers themselves. They must be able to demonstrate that their reports have been effective: most will provide a list of clients and testimonials. And when you check, you’ll find those testimonials corroborated elsewhere, by people who are not in the agencies’ employ, and who have had a good degree of publishing success as a result of the advice that they received.