Absolute first thing that popped into my head. There's a similar quality in the voice I think, despite different ranges. I don't know. It's a stretch to compare your voice to anything *I'm* familiar with.
Your voice is much deeper/lower though, obviously. I think it's more Portishead's style and how her voice is very 'real' ... and the fact that your demo was essentially recorded 'live' and not tampered with. If you'd gotten the.. .say... Britney Spears treatment, I probably couldn't make a Portishead comparison.