With BA at Heathrow there should be no reason for a tag to be stuck to the back of your passport. If you drop a bag at a machine then it will spit out a till receipt style tag. If you drop a bag at Club check-in or First wing then the agents should be sticking the tags on to a way-finder leaflet where it says "attach bag tag here". If you aren't given a tag on this then you should insist on it if you don't want it stuck to the back of your passport.