The judges did create this problem (either deliberately or not depending on whether your wardrobe consists of mostly bacofoil) by creating a joint 1st place scenario.
(the following assumes a 1, 2, 3 placing)
The great unknown is whether Tom had the most phone votes or not; if yes, then the judges would have been choosing between Lisa & Rachel in the dance off, and Tom would have been on course to win and Brendan's calculations would have been irrelevant.
If he wasn't topping the poll, then he would have been in the dance off and out. I sincerely hope that this wasn't the case and that he doesn't go on to win on the basis of some weird sympathy vote, but the BBC/producers/whoever needs to anticipate these situations and prevent them from happening