I don't know what I would have done in Maddie's shoes. The odds are against her (or any of them) winning, so £3k in the hand is worth having, but a potential £120k is life-changing, and there may be media opportunities for the winner if they want them (eg appearing on The One Show or This Morning). £3k is very nice, but not life-changing.
OTOH, she is young and has more life ahead of her, so it could be that it would be life changing for her. We don't know the circumstances of the people involved. I thought the responses of the others to her choice was interesting, too. Again, £3k is a lot of money, but as a percentage of £120k it isn't, and getting rid of another possible winner has to be of greater advantage to the survivors, yet they all cheered her decision to stay.