�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 chosen to chime with the 'f' of a strong swear-word, with the �of our stairs' just tagged on to make sense. The whole thing is, in other words, just a polite version of "Well, I'll go to f*&@!"
I can remember my grannie saying 'Well, I'll go to the foot of our stairs' as well as 'Well, I'll go to Wigan!'
But I can't for one minute imagine her using that phrase if it actually meant the other F word!! She'd have used it in all innocence (God bless her!!)