How do you tell a real dissertation editing service UK from a glorified proofreader?

I've reached the point where the draft is done and I need to decide what comes next, so I'd welcome some perspective from people who have already been through this.

The confusion is that editing means very different things depending on who you ask. Some providers describe it as proofreading — grammar, punctuation, referencing. Others include restructuring, argument coherence, and feedback on whether the chapters actually connect. All of it gets advertised as a dissertation editing service UK, but they are clearly not the same product, and the pricing reflects that.

What I'm trying to work out is what the service should reasonably cover at this stage. Is it worth paying for structural feedback once the draft is written, or is that something that should have been resolved months earlier? And how do you tell, before handing over both the money and the draft, which kind of editing you are actually buying?

Any experience with this would be genuinely useful.