I work in central London and drive up and down every week so that would not have been a problem.
I feel I don't know enough about ducks to know whether it would work for us, I can put a pen together next door to the hens easily enough, would have to think about finding a duck house though, but if I let them free range during the day do they know where their "home" is and come back to the pen.
We have fields around so when the chickens free range we do keep an eye on them but they keep well within our boundaries, however the spring fed pond is down by the boundary so if they head there they could head off into the younder and if they don't know to return that would be an issue.
Sorry for picking your brains but I would not like to offer them a home if I cannot look after them ok.