Private mortgages?

se2020

I was previously looking at a property (part finished barn conversion) but I was unable to get a mortgage on it as all the lenders my broker (and myself) tried would only offer self-build mortgages and I do not have the income to support the final value of the build. My plan was to move into the finished part and do the rest of the conversion  myself over a few years funding from my own income.

After lots of searching and not quite being able to find a lender the broker has suggested a private mortgage,  I thought this would mean I would need to find family or an individual to lend me the money but apparently private mortgages are a bit more mainstream than that.
A google search does indeed show plenty of results ranging to private banking from big name banks to companies that I have never heard off.
Does anybody know anything about these?
Ideally a rough idea of the rates they would charge as well?
Thanks!