Unfortunately it all depends on the class of anchor and material being fixed to. Key things are to ensure anchors conform to BS EN 795 & are installed to BS 7883:2005 (code of practice covering selection, installation & use).

Surely it depends on purpose of the anchor? Fall restraint vs fall arrest. The British Standard has its place but its very non specific to your roof. So still need to apply and justify via a more practical risk assessment. Can you reach anchor point safely without putting yourself at risk? Is it fixed in an appropriate location to enable fall restraint lanyard to be effectively used or is misuse/abuse foreseeable? If fall arrest, ideally they won't be into the roof surface being stood on, they'll be at a higher point to reduce the overall potential fall distance and the loading on the lanyard as it goes over the relevant edge.