As you say, I think it's just the slightly different legal systems. Part P is in the Building Regulations for England & Wales, but Scottish Building Regs. are somewhat different.
As I understand it, Scottish Building Regs. have specified that installations must follow I.E.E. Wiring Regs. (now BS7671) for a good many years, whereas in England & Wales there has never been any such requirements (and still isn't).
Part P doesn't apply in Northern Ireland either, which again has slightly different laws.
