There are two transactions here:
A loan (or possibly a purchase of shares) from the investor to the company.
A loan from the company to their customer
They are not related. As far I as know, there are no major laws concerning the payment to the company. A company making loans to consumers has loads of regulations under FSA, Office of Fair Trading, etc.
BTW, there reason they would not use a bank is that it is likely they are poor credit risks, and so pay a higher interest rate, allowing the company to pay a better dividend to their investors than they would get at a bank - but at a higher risk.