It was common in Communist countries for workers to be forced to work without any form of compensation (however it was done in each system) for a day or two each month, so the workers could show their commitment to socialism or some garbage. Still to this day it harms the idea of volunteering/NGOs/charity/civil society in some eastern European countries. I doubt the right wing newspapers would've been keen if Corbyn proposed this.
If people want to volunteer they can, if people want to join the military they can. If neither is happening, it's because the conditions are shit and it's not worth it. My guess is that if young people aren't interested in volunteering it's for similar reasons to sports clubs shrinking. The economy has gone to shit and especially so for young people, which makes it difficult maintaining what amounts to an additional unpaid job. Even things like the minimal cost of travel to the de facto unpaid job starts becoming an issue. Someone isn't going to be inclined to give anything up if they have nothing to spare.
The Tory's want indentured labourers to fill all the structural holes in the state and private sector they've created. Crazy how often the NHS has been mentioned in connection to this manifesto pledge. Unskilled people with no experience who are unpaid and forced to do the job, are going to be a burden they're not going to fix the NHS. It's all completely mad, the UK is a developed country with high levels of tax, there shouldn't actually be any need for a volunteering sector or food banks or any of the rest of it.