I passed phase 4 in March and they allowed me to postpone my departure until August to enable me to finish my degree. Having said that, they knew I was still in my final year at University when I applied so they must have known I would be wanting to finish it.
If backpacking is your reason, rather than something like a degree which you're already committed to, then I don't know. The application form does give you an option to specify what is the earliest date you would be available for training (if I remember correctly!). I would say definitely go for your phase 2 resit on one of the dates allocated, and then have a word with the staff on that day. They're the best people to ask.