HB, the exams are only to be used for official purposes. Hence the PPL(A) exam papers may only be used where a mandatory requirement exists; nowadays this means for initial grant of a PPL(A).
It is not up to some flying school CFI to decide that it's a good idea for someone renewing a PPL to use the official CAA exams for that purpose. If you don't believe me, ask PLD Approvals Support.
Nothing wrong with a CFI writing his/her own Air Law exam if he/she wants to - but the official CAA exams may not be used for this purpose.