It's a bit of a shame that no-one has had a go at this...
Firstly, if you are in a trade that is in short supply, forget it!
Secondly, if you have been warned for deployment, forget it, although you can sign off!
In both cases, you will have to soldier on and serve your time-bar. Remember that, in normal circumstances, you have to give 12 month's notice to quit!
However, now see your Tp Comd or equivalent and explain the situation. Ask to see the RCMO - or visit him yourself. Lay your cards on the table and explain:
Why you want to quit?
That you have thought through the implications? (Have you?)
What you are going to do outside? (join the police - good, go clubbing in Ibiza - bad).
Where you are going to live?
On the upside, it is possible that you will be allowed to leave despite the time-bar. On the downside, your testimonial might say something that will make life difficult in the future! For that reason, you might wish to read QRs - in the area of para 9.414 - these paragraphs explain how one might be discharged from the Army. Most of the paragraphs cover mis-behaviour or pregnancy but none of them cover "suddenly waking up and realising that I don't want to soldier on for less than the minimum wage in an operational environment"! Be aware that even SNLR can be adversely viewed by civilian employers!
If you have a guarenteed job to go to on Civ street you can apply for early release through MCM DIV, took me two weeks to get out! Downside being I forfeited my re-settlement and lost all of my leave, but if you want out and have a job outside you just need to justify that there is no chance of that job coming up again and that is great oppurtunity blah, blah. They can only say yes or no, worth a try! Good luck!