�Well, I'll go to the foot of our stairs' is a euphemism for a much stronger expression of surprise, just as �Cor blimey!' is an acceptable version of �God blind me!' The word �foot' was probably chosen to chime with the 'f' of a strong swear-word, with the �of our stairs' just tagged on to make some sense. The whole thing is likely, in other words, to be just a polite version of "Well, I'll go to f*&@!"
My grandfather (and my father) used to say it; also my grandfather used to say "Well, I'll go to our house". No doubt there's some arcane etymology lurking behind that somewhere!