Sadly, you have absolutely no real way of choosing where you are posted. It really is done on business need and where you would be best suited in terms of the work, where you're likely to do well and validate, not which location would suit you.
There's a guy who's just gone through college whos life is in Aberdeen, girlfriend is in Aberdeen, held at Aberdeen between courses, the Manager ATC of Aberdeen really wanted him to work there - and he got Cardiff.
You, of course, may equally get exactly where you want but it's russian roulette at the best of times and it changes every other minute as regards where needs people and how badly.
Obviously also the different disciplines will direct you to certain places. If you get sent onto the area course then there's only really 3 choices of where you can be posted (Manchester/Swanwick/Prestwick) - but if you get sent onto the airports courses then clearly the choice is far more widespread. As regards whether you can choose to do the airports/area courses - you can make a preference but again - it's a business need decision. There are people going through at the moment who made no request onto the airports course, but got them anyway even though they presumed, and were told, that if they made no request onto airports they were going to get area.
So there you go!