Laying decking beside a house

I am planning a decking build that will span the length of our house (approx. 8 metres) and extend approximately 2.7 metres away from the house. I would also like to construct a shelter over part of the decking though to enable use in most weather conditions.

The issue I have is that the sewer line for mine and the neighbours house is below the surface I plan to deck over, and I cannot dig down the recommended 150mm for a hardcore and sand base.

I have read about using postcrete/concrete to create plinths that a floating decking can be laid onto, but would like to know if anyone has knowledge of using 75mm steel post supports/holders instead of postcrete/concrete. This would, I think, provide a rot-proof foundation for the floating decking, but also allow the upright to continue straight up onto the roof area.