As far as i remember you don't get your posting order until near the end of your course, if they still have the same system in place then you will fill out a preference of posting form while you are on your course, then probably not get any of your choices.
Discounting 42 Geo Regt and units yet to be fully formed, there are 7 Regiments in England, 4 in Germany and 1 in Northern Ireland. Without boring the pants off you, 7 of these Regts provide close support to Brigades, 2 provide general support at Divisional level and 3 are specialist units (ie EOD, Air Support and Resident Northern Ireland).
I wouldn't get your hopes up too much for a particular Unit - I put down Antrim after my B3 course in 1995 and was sent to Hameln - but lots of people at least get a posting in the country they want, if not the exact location they wanted. Just stick down Germany and hope for the best