Hire a solicitor to argue that as your salary is paid monthly, that should be your term of notice and threaten the European Court of Human Rights if they disagree. you'll be out in a month just to keep you quiet.
there is no quick way- unless you're really desperate then take a class A drug and declare it to the reg staff( actually any type of drug) - then spend a bit of time on holiday and hit civvy street!!!!!!!

Failing that put in your notice - if you find a job and get an offer the mob may give you early release - dependant on branch/ unit/ etc........