The item of information which I have just heard is so exciting that I need to go to the foot of the stairs in order to shout it up to tell the people upstairs. (If it were even more important, I would actually run up the stairs; if it were less important then I would stay here and wait until the other people come downstairs before telling them.)
That's what it means, although I don't know the original origin.
Just as 'Cor blimey!' is a euphemistic (ie more polite) version of the oath 'God blind me!' and 'Crikey!' is a euphemistic version of 'Christ!'...so I believe 'I'll go to the foot of our stairs!' is a euphemism for another saying that opens: 'I'll go to' and goes on to another short word beginning with 'f'. I'll leave you to work out which word that is, Ann.
This phrase is used (Lancashire/Yorkshire) to convey astonishment OR sarcastically to pretend to be astonished by something which is totally underwelming. If someone says something which is blindingly obvious, then the response is ...Well, I'll go to the foot of our stairs. No idea where it comes from though.