it probably did but its easier for them to just wait until it does fail rather than send people out to replace batteries that might last a few month longer
you could use a 10mA RCD for the washer which will trip first at lower current however in case of a fault then chances are both will trip at the same time or just the RCD in the board
just another reason RCBO boards are better
whatever you fit will help, but they will not be enough on their own
you'll use pretty much whatever you generate in real time so no point storing it since there won't be much leftover to store
people would rather spend money on something they can show off like a new kitchen or bathroom, rather than spend a few £ on something that could save their life / stop the house burning down. the old one works so it must be safe...