Like you say it's a personal decision but if you knew that a preventitive treatment would most likely significantly increase the chances of many more years with your loved ones you'd take it wouldn't you?
I'm sorry to hear about your nan.
I wouldn't have the treatment unless I had the cancer. Just because I may get something (even if its high 90s % wise) I still wouldn't go for it. Maybe I'm too selfish or vain or whatever but I really couldn't get rid of my boobs on the off chance. It would have to be a "you're going to die unless..." Thing.
I don't want children so have less of an issue with thought of getting a hysterectomy.