destinyapple,
the question u have post i will not be able to answer u.... as for
who is suppose to serve NS in police, army or even civil defence it
will only be sort at cmpb......
thou there are rumours like :
1) 1st guy in family will usually go commando, or
2) if u join NPCC u will get into police force, or
3) even which month the letter is send for you to enlist will have
higher chance etc etc...
but in the end still lies in the database in CMPB to sort out who
to go where.... usually those whom qualify for serving NS in the
police force will definately have his family screen by SPF as one
of the criteria....
perhaps u might want to consider of join the police force as a
regular.... 
