http://www.microgenerationcertification.org/images/PV Book ELECTRONIC.pdf
There are guidelines rather than rules - the important thing is to be able to prove your design will take the loads imposed by wind.
Anything less than 500mm puts you in the edge zones which requires more careful design.
In general you can be 300mm away from the roof edge without the design becoming ridiculous - this is dependent upon the roof and the situation. A hipped roof will experience more turbulence, and one next to the sea will require extra care...etc.
How tall is the building? A single storey garage in a sheltered location will be more tolerant than a 4 storey house by the sea.
2 other factors to consider are:
1) Lifting
tiles to fit hooks can damage mortared ridges and verges. A rule of thumb is 300 at all edges for plain
tiles, 300 at the gutter and 500 from edges and ridge for concrete Redland 49 type and large format modern interlock will possibly require 600 from the ridge. Not a problem for dry ridge and verge construction and slate roofs of course.
2) going too close to the gutter with a bolt-on system can lead to water overshooting the gutter and causing damp issues. 300mm min on a 45deg roof is a good rule of thumb.