You were cleared to a fix, descend to 8000. Do that.
You were then cleared for the approach. The fix to which you are currently cleared happens to be the IAF. Your clearance before the approach has not changed. Proceed direct to that fix, descending to 8000. Descend from 8000 only after established on the approach (i.e., passing the IAF) or specifically cleared for lower by ATC.
I have not seen any rules written anywhere that are different from the FAA rules. What have you seen WRITTEN that gives you the impression you can descend below 8000 enroute to the IAF when only cleared to 8000?