Conscription would compromise the effectiveness of our all-professional armed forces, and it might promote a bellicose foreign policy in order to give all those young conscripts something to do, and thus justify the expense of keeping them.
However, some form of uniformed, disciplined, barracked service between 18 and 21, doing good works at home and abroad, would be no bad thing at all. While people might be doing anything from the very academic to the very practical, they should all be inside the system until the age of 18. They could then go into this.
After that, for example, universities might admit at 21, which would have all sorts of beneficial effects, just as it did in the days of National Service.