Push/Pull is a general rating of the nation. Europe has a base of -1, while the Americas have a base of +1. Asia and Africa have a base of 0. Since there is a blockade, the CSA is currently getting immigrants from only Mexico (RE: Little to none), while the USA is still getting a nice stream.
Looking at immigration, the UK has -5, and the USA has +1. That means America has an overall rating of 6 with British Migrants. They will go to whoever has the highest.
OOC: Ah ok. I was wondering if that was how it worked. Makes more sense. (I was really struggling to imagine a slow liner packed with thousands happily sailing past warships ready to fire upon them )