This used to be in the appendix of the approved document Part B, as examples of construction satisfying the B Regs (an idiots guide!), they no longer do this so you have to reference manufacturers information and verify its been tested etc. It was probably a hangover frpm the "deemed to satisfy" clauses from the older format pre 86 regs.. The examples I have quoted have been independently tested to BS 476 for a FLOOR situation.
Obviously a floor test would gives different results to a wall test, the certification makes this very clear. I only referenced the website to back up what I am saying is correct.
Can you reference where it states a "rule of thumb" is acceptable evidence of periods of fire resistance? Your rule of thumb did not state any thicknesses or number of layers is required, so presumably a single layer of fire line gives 1 hour and 9.5mm plasterboard gives 30 minutes, according to your "rule of thumb".