Yeah, you can't just refuse to take part in the war.
If things would be so easy you can bet that there wouldn't be quarter as much war.
Brits are important on the political, economic and all around a big player in the world.
If WW3 breaks out they can't just stand around, not with the current globalized world.
If all you need to survive a huge scale war to just refuse to participate in it then you can bet how many people would choose that.