Im planning a basement conversion. The external ground level is ~1m above floor. I intend to line the walls of the basement with Newton 500 or similar, and have basedrains at the bottom leading to the combined sewer. Fall is tight I think but I haven't excavated outside yet to see how deep the clay pipes are. Advice needed as I've never done this before.
I would be thinking along the lines of a pump system, that way you needn't worry to much about drain levels, also if you had trouble with the combined sewer system you wont get flooded with shite.
You need fall, a trap, an anti-flood valve and the gods on your side.
If your system or the local authority system blocks then water under pressure will find its way into your cellar, It is of course possible but as stated above you need a site specific survey.