You say something to a boot when you want to be humble. You're down there because you're bowing in front of the person you're humbling yourself before. Because of that, your face is really close to their boots and you're speaking directly at them, instead of at the person's face, so it's more discreet.
You're being humble like this because those boots belong to the King (きん) of England. You can't just say whatever you want at eye level to a king. He'll have your head chopped off for insolence, or worse!
Currently there are no similar looking kanji which exist in the WaniKani learning system