Parliament can't over rule the EU - the extension is still there, in EU law, to be requested.
That we've legislated against our ability to ask is simple posturing. We'd have to re-legislate in order to be able to ask, but that's within our power. Not, I think, that we're going to do so.
I think it's full speed ahead, fuck business and smash into the 1st of Jan 2021 with a fist-full of blue passports riding a wave of British Fish.