Yes that's the awful thing about anxiety - it plays on the mind and makes us want to retreat to 'safety' in a familiar place. Do you have BMs at a certain time? Maybe if you alter your eating times you can alter when they occur, although I think most people have at least one BM at some point in the morning. I'm guessing your city break is not a long flight, so you won't be on the plane for too long.
Do you do any breathing exercises or anything to try and calm down when you have anxiety symptoms?
I started re-taking VSL#3 probiotics yesterday to see if it has any effect. I wanted to ask how soon before you have the BM do you start experiencing the anxiety? Mine appears to occur when stool or gas is passing through the end of the sigmoid colon - which is the area I always seem to have symptoms in with Ulcerative Colitis.