Best of luck with whatever result you get, no harm trying for an alternative solution, however at the end of the day it can be very hard to change someones decision once it’s been made.
If it doesn’t work out the way you want, I wouldn’t worry too much, the role or one similar will come back up and you can apply for it at a more suitable time, one that works better for your time frame.
If you really do want to leave, submit your notice, work out your last day, take off all the absence you’re going to get and use this as your earliest start date, take three months off for a reasonable notice period and a couple of months for the application process and this becomes the date when you can start looking for a job, safe that you”ll not have to worry about a cranky boss refusing anything.
There’s always been a debate about when you can start being paid, I think if you’re having a placement it could be an issue, however, once you walk out of those gates for the last time I don’t think anyone is going to really care, as long as it’s not a job on your old unit or with a entitlement to a Government backed pension.