When the UK last had compulsory military NS there were garrisons all over the world. What would a new generation of soldiers do? Is there any point in training them up only for 99% of them to leave as soon as their time is up? That would waste the time and (stretched) resources of those who have to train them.
The forces would do better to attract recruits who want to serve, rather than pressganging a generation that suffered disproportionately during Covid and in most cases will have no desire to join up. It makes no sense to on the one hand refuse usual entry channels to, for instance, people with tattoos, and then coerce them into joining on the other. Would tattooed young people who decided they did want a career in the services after NI be turned away, or their progression limited? Daft rules like that need to be sorted out if the forces want to recruit willing soldiers (who are, as the saying goes, each worth ten pressed men).