I think you need to know what all the options are, first of all. Like, what is the worst case scenario if you refuse to go if/when the British Army calls upon you? If it is jail time, how much jail time? Would the jail time be more or less hardship for you and your fiance than reporting for active duty? Are you willing to accept the social backlash, and the potential problems with current and future employers? Are there others to talk to who have been through it themselves?

I'm in the US, but I'm guessing there may be at least one or two good organizations giving legal advice to potential conscientious objectors in the British military over this. Here is what I found in a quick web search:

From War Resister's League:

Information for Members of the British Armed Forces

Sad to say that according to this, you are among the group which is most likely to be called up as a reservist. I know it isn't much information, but it is a place to start. One of the better things about the CO application process is it can take quite some time to get through it. If there is a war, and it is short, you will benefit from the prolonged process.

Good luck. It is a tough decision, but if you feel a strong (and I do emphasize strong, as it will have to be to get a CO exemption) moral reason for opposing the war, your chances could be excellent. So--that is my advice. Go for it, but go for it with your eyes wide open.