When you talk to 'bespoke' boat builders, some are more bespoke than others - some will try and convince you that they would rather build you the boat they want to build, rather than the one you want built.
On the other hand, most of them do know what works and what doesn't so it's all a bit of a balancing act. Do talk to as many owners as possible to find out their experiences on what worked, what didn't and how was their relationship with their builder (are they still talking?!). Walk the towpath and talk to owners of newish boats.