The law states that all lenders must show the APR so that potential borrowers can see the cost of borrowing. That is fine for mortgages and personal loans of a year or more, it is an essential easy comparison.
However, this law also applies to very short term loans of a few days or weeks where APR is of no use whatsoever.
If a company agrees to loan £100 for two weeks and expects £110 back most people would agree that is reasonable - there is no security and the borrower is high risk but the APR works out a very high Annual Percentage Rate even though the loan will never be over a year.
These companies would never offer loans of 1 year, let alone 5 years.