I wonder what they would have done if the young girl really had been the only one left, with a prize fund of zero. There might be something in the "rulebook" to cover this, then again there might not, it being so unlikely to happen.
I doubt they could have a Final Chase with a prize fund of nothing at all. That would be just silly.
I suspect either
A) They would put a token amount in the prize fund, say £500 or £100, or
B) They would treat it as an all four eliminated situation, and the team would get to choose one player to come back to play for a thousand each.
Maybe somebody from Chase HQ will come on here and tell us.
And while they're here they might also tell us why the show is getting so predictable, with never showing two wins in a row, the weekly win is nearly always in the middle of the week, and even the Chaser that loses each week following a fairly predictable pattern.
I doubt they will, though.