I don't know the full story, but I would imagine that she knew that she was taking a huge risk and what the penalties are for drug smuggling in Indonesia.
From Wiki, it's not as if she hasn't had support from UK authorities:
"Sandiford subsequently launched an appeal against the court's decision. She had funded her own defence costs during the initial trial but had no money for representation at appeal. Funds were subsequently raised to pay for an appeal lawyer, and the appeals procedure began. In the United Kingdom lawyers applied to seek a judicial review of the government's stance of not providing financial aid for Britons facing criminal proceedings overseas, but their legal challenge was dismissed by judges at the High Court of England and Wales on the grounds that Sandiford had little chance in successfully appealing the sentence. An appeal arguing the government's position was unlawful was also rejected. The British government submitted a statement to the Court in Bali alleging unlawful behaviour towards Sandiford by officials at the time of her arrest".
Not nice to think that someone is going to be put to death though.