With respect, Finland and the UK are not alike at all.
"Will", yes fundamentally important, otherwise all you are doing is compelling a bunch of unwilling, demotivated people to do something against their will, which is not going to be conducive to the young people getting anything out of it, or the service they are supposed to be a functional part of fulfilling it's role.
The UK is a political State comprising four constituent nations. It didn't exist prior to 1707, and didn't exist in it's current form until 1801. People from the UK might describe themselves as "United Kingdomites", but in reality it isn't a nationality and the UK is not a "country" in the typical sense. For quite a while now Independence from the UK has been the preference for at least half of Scots, with that preference even more strident among Scottish people below the age of 24. How do you propose to motivate Scots youngsters to perform "National Service" for a State that the majority of them wish to see the back of? Do you think Republicans in Northern Ireland are going to willingly oblige?
There has to be some sort of feeling of ownership and participation before people will gladly think about sacrifice for the State, and for a large proportion of people in the UK, the UK prompts no such feelings.
Would I actively defend the place I live from a hostile invader? Of course, but for the simple reason I'd like to protect my family, my friends, and my compatriots. Couldn't care less about protecting the UK State though, and similarly, if it tried to compel me to participate in any form of NS, I'd opt for the prison cell at the UK State's expense instead.