Brexit hasn't even occurred yet. It's just talk thus far. They will still need the low paid migrants to do the jobs British people don't want to do. They will still want to attract the skilled workers. In addition it will do absolutely nothing with migrants from outside EU.  
Just like in Canada, who do you think works in agriculture or other menial jobs? Just last year or 2015 alone Canada had 500k temporary migrant workers!

Immigrants have always been the easy target for politicians. However immigration was NOT the main point of brexit campaign. It's more likely the other way around migrants from EU in UK have benefited the UK economy greatly. Why worry about investing time and huge amounts of money on raising people (health, education etc) when you can just import them already grown up and educated?

As for the armed forces reporting to feds or individual states, I don't see the point you're making. Are you making a point EU should have federal armed forces?