To be honest I actually don't think it sounds all that bad. If it were forced military service I'd think it was terrible. But it sounds like you can just do community service instead of the military training. And it's just one weekend a month for twelve months. Given how much we're disconnected from our communities these days I think making sure kids engage with their communities in this way could actually be really beneficial.
That's just thinking about this one policy in a Vaccuum though. I know nothing about British politics so I have no clue what other awful and heinous shit this party may or may not be into.