John, grey waste is grey waste, not poo.
The conversion of your rain pipe hopper will still illegally send poo in to the grey waste system.
Poo must go to the poo waste stack or to the poo waste pit via a new pipe, using a Saniflow system breaks down the poo so it can be pumped via a smaller pipe, it MUST still connect to the poo waste (NOT the grey waste).
When the water authority realise that you are sending poo via the grey waste they will investigate the contamination source and fine you with costs (of the investigation and the treatment of the grey waste that will have poo in it). Sewage in the road side is x 2 pipes, one is the open system for grey waste and includes street gullies and all rain water waste.
Poo sewage waste is a separate sewer pipe and (obviously) is treated more extensively at the sewage plant. Currently you are causing the contamination of the local areas grey waste.
So PLEASE get away from the thought that you can easily remedy the problem by changing the hopper, the solution is to re-plumb the poo waste by connecting it to the existing soil stack or soil pit- You have no choice.
Regarding the illegal loft conversion, you may get away with it being classified as a loft space, a playroom or storage. Under no circumstances will BC allow it to be sanctioned as a bed or living room space due to fire issues, means of escape, stair size (possibly), fire detection system, insulation and a myriad of other things.
If the BC are due round, I would suggest you tell them that the room is not permanently used and that your reason for visit was to get an understanding of what would be required to convert it to a habitable space.
Please report back to us all on what they say, I'm sure the DIYnot massive will be pleased to assist further.