If the main requirement for a CC licence is to navigate around the system I struggle to see how this is realistically compatible with all the work involved with fitting out a boat. Although I fully accept that it is not impossible and others may have done this successfully.
ETA I'm not having a dig, I genuinely don't know how anyone could do it. Just the logistics of getting materials, using power tools, storing everything needed while working etc. I get that not paying for a mooring saves several thousand pounds (that can instead be spent on the boat) but the inconvenience of navigating while carrying out major work would be way beyond what I could imagine.