I have this setup in our house at the moment, for a toilet on the 2nd floor. Only difference is the distance between the first 90 deg bend and the second is about 7ft. It took some time to get the right fall as per the reg's - I went near to the max recommended but not beyond it (90mm/m iirc) in order to keep momentum but without leaving anything behind. In the end I lowered the ceiling in the room below by about 75mm - fortunately in our place that means the ceiling is "only" now 2.6m in height. So far its been in 9 months with no issue.
Not sure what a building inspector would make of it - but not really bothered either as long as it works
Then again, we also have a macerator toilet installed in our place (the horror!) which, if you believe the internet means our house will be entirely consumed by effluent any day now
Admittedly, looking at it again, the drawing I did shows a slight upward slope on the section between the 2 bends - something I would NOT recommend!