To build in what McC says, it works as follows.
The company will advise you of bases available should you be successful. IIRC you are asked to state your preferences on the recruitment day.
You will then be offered a contract at a base.
On day one on your induction, two things happen. Firstly you get a seniority number. Seniority is allocated by date of joining and the individual course is separated out using numbers from a hat.
The second thing that happens is you are asked to complete a Standing Bid form. Whatever you do, think about this and bid for what you want and what you are prepared to accept. The reason for this is simple; although we run a full formal bid around July for next summer, people retire, resign, die, lose medicals etc and casual vacancies are filled by going down the seniority list until we hit the first bod that has bid for that base / fleet / rank combination. We've had new joiners into their base of choice within months of joining, including very popular bases. We've got people going on the 756 for Summer 16 with very few years behind them.
I've lost count of the number of people who've rung me, fed up because someone junior to them has just got what they wanted. 9/10 it's because "I didn't think it would be available". Slightly fewer are the guys who bid for something and then changed their mind but forgot to change their bid. We've been able to accommodate it usually because someone else lower down wanted it but if they hadn't, you're stuck with it!
Anyway, short of running a full induction course, I reckon that's about it from me. Thank Christ I hear from the naysayers.
To the pub!