Nobody would suggest that comparing the rates charged by lenders of last resort to MLR is sensible however the rates charged by credit card companies are generally considered fairly high and are easy to access for comparison. Granted, "payday" loans are high risk and deserve a higher reward. But is 100-200 times higher justifiable? I just ask the question. In the case of Kettley v Scott [1981], the court stated that consideration would be given to the prevailing interest rates at the time, the age and experience of the debtor, the risk involved and the degree of pressure brought upon the debtor. It would be interesting to see the attitude the court would take to a challenge to an APR of, for example, 1700% or 4,000%.
As for APRs, their use was adopted, if my memory serves me correctly, in the 1970s under the Consumer Credit Act to allow for a fair and reasonable comparison of interest rates for all types of borrowing and they take into account the amount lent, the interest rate, the period of the loan and the periodicity of the payments. There are standard methods of calculation which take all these factors into consideration for all types of lending. I do not understand the point made that "APR is only relative to the length of the loan". APRs are the legally acceptable method of comparison of the costs of credit.
My understanding of the term "opportunity cost" is "the cost of an alternative that must be forgone in order to pursue a certain action", so I am not really certain I understand the context in which it is used here. Do you mean the cost to the borrower of not borrowing the money compared to the cost of borrowing it? Or the cost to the lender of not lending it and using the money in some other way? Sorry if I'm being dim.