I dunno. 12 stone seems really reasonable. With too much weight, the new area won't get the same work, will be put under too much stress and it makes post-op significantly worse.
About the ceramic replacement, that's unfortunate. typically when they break the femur to attach any synthetic bone material, they don't take into account what was lost in the "break" and if they need to grind down/polish the femur end, if they don't account for the bone loss, the leg will be shorter. I recall seeing some photos of bad outcomes where the person lost more than an inch on that leg and had to wear orthotic shoes just to walk.
Hopefully the weightloss goes well (in older folks, anaerobic exercise....muscle building...works best. Stay away from cardio. Even small weights can make a big difference in activating dormant muscles and the more muscles turned on, the more the body has to metabolize to support.