I don't support this policy or national service in general, as I don't think it necessarily benefits either young people or the institutions facilitating the plan.
In terms of young people; I don't think young people should be forced to do military service or volunteering. For clarity, I'm not opposed to the idea of these things being promoted, perhaps in a similar vein to how National Citizen Service was promoted to me and my cohort when we were in our GCSE year. If young people would like to do these things, I would absolutely be supportive of the government helping young people to find pathways into the military or into community volunteering and helping them to sign up. However, I am opposed to them being mandated. Mandating things like that could impede the career progression and post-18 plans of some 18-year-olds, and even putting that aside, the idea of forcing things like that, with threatened "sanctions" for not "complying", does sound a little draconian to me. Also, isn't volunteering, by very nature of the name, supposed to be voluntary?
In terms of the institutions facilitating the plan; national service lumbers them with a load of 18-year-olds who don't want to be there. Surely the military don't want to be lumbered with training a load of 18-year-olds who don't want to be there and are purely doing their year to tick a box? I can imagine that an unmotivated army is not an effective one, and surely all any country wants is an effective army? In terms of the community volunteering end, I don't deny that institutions such as the fire service, police and NHS benefit hugely from volunteers. However, all of their current volunteers are people who do their work enthusiastically and consensually and have specifically elected to volunteer at that institution. If you throw a load of 18-year-olds forced to do volunteering against their will into the mix, many of these people will not be enthusiastic volunteers and will be there simply to tick their national service box. At that point, I can imagine the whole thing becomes more trouble than it's worth for the institution running proceedings, and they'd almost have to become de facto schools in terms of managing a load of young people who have no desire to be there. I'd imagine that no institution with volunteers wants that.
It does seem as though more people support national service than you might expect, though... the Tories are claiming that 57% of the country supports national service. It definitely seems to appeal to some older voters in particular.