Apart from the very few exceptions mentioned including Dancing On Ice I don't think once you've been in the bottom 2 you can expect to win as it shows there's no big public vote for you.
Once Emma was in the bottom 2, I think for a total of 3 times before the semi, I knew no chance, along with Zoe.
Darren was never in the bottom 2, his public vote was way ahead of everybody else's in the third series, once the dancing kicked in.
Mark was never in the bottom 2. When he and Karen were called out first for the final, it was just a matter of tying up the dances.