I'm in favour of conscription for two reasons. Firstly, it would introduce people to the Forces who would not currently think of joining - some of these may actually enjoy it and want to sign on either full-time or part-time, thereby increasing recruitment.
Secondly, one of the most frequent excuses given for crime is boredom, closely followed by unemployed. Military training will certainly prevent boredom, while a well-structured conscript training schedule, offering trade training rather than just basic training, will help to increase the employability of those who choose not to remain in uniform. It should also be remembered that conscription would catch a lot of people before (i.e. instead of) turning to crime.
In the event of a political blunder that requires extensive call-outs, there will also be various levels of "ready-reserve," (maybe not quite "ready," but requiring a shorter work-up than a new recruit), depending on how long it has been since people have been demobbed.
Conscription should, IMHO, take three forms:
1. Regular Conscription, where people serve for, say, 2 years full-time.
2.TA Conscription, where people commit to a TA style of service (probably with a mandatory higher number of weekends in the first 2 years) for, say, 5 years.
3. Penal Conscription for naughty people - similar to Regular Conscription but in some nice, out-of-the-way, place like FI with duration determined by the courts. Gets them off the streets and frees up jail space. Could also be quite effective for drug "victims."