No Ben, Building Regs haven't changed - not in this regard. You are probably thinking about Planning Permission.
The change in PP impacted the rules for Permitted Development, but if it was OK before under planning guideleines, that won't change by changing the roof.
What WILL definitely change is that it needs Building Regulations approval - and particularly it will need to satisfy the thermal requirements, and the calcualtions to show the loading on the roof will be needed.
Oh, and the roof structure and walls will need to be stronger because tiles are very weighty.