I held off saying anything for a while, because I'm not the self-publishing guru, and there are plenty of them around here, but you're certainly in the right place for lots of help and advice.
One thing I do know is that the competition in self-pub is no less cut-throat than in traditional publishing, and you need to make your manuscript sparkle as much as possible. First question: who has read the manuscript, until now? In my view, the key to improvement is getting as much feedback as you can from as many readers as possible, and while an editor would be a good idea, I'd start off with peer-to-peer critiques, either here or on another forum (Goodreads or https://www.critiquecircle.com/ perhaps?). People are often open to the idea of swapping chapters (I certainly am!) so that might be a good way of getting started. Let me know if you're interested.
Either way, making the most of other readers' opinions (even if those readers happen to be other writers) strikes me as the best approach, before going for a professional editor.
What kind of novel are you writing? If you can give us an idea of genre, maybe you'll pique the interest of others writing in a similar area?
To give a literal answer to your question (finally), Jericho have their own editing service, and I can certainly recommend the wonderful Emma Cooper (https://emmacooperauthor.wordpress.com/) who did a brilliant job of producing a manuscript assessment for my current WIP (which I'm still working through), and while I'm talking about successful authors who are also editors, I have to mention the highly perceptive Laure van Rensburg (https://www.laurevanrensburg.com/critique-service). These two represent rather different genres, I have to say, so it's worth thinking about where you sit, and where you'd get the most benefit.