Many migrants actually send most of it back to their native country so I'm not sure how that is going to help the UK economy?
Now you're starting to use a bit more logic. That's good!
Yes you're correct that some migrants do send some money back home, and that of course won't help the economy in the UK. That's one of the 'secondary effects' I alluded to in my post (though I didn't mention that one explicitly).
I'd be very surprised if significant numbers sent most of their earnings back though - that would be quite difficult by the time you've paid for food and lodgings in the UK. More likely, it's
some migrants sending
some money back. The effect in terms of employment levels in the UK is likely to be fairly small (though I'd accept, not zero).
However, I would imagine the migrants most likely to send most money back aren't the ones from the EU - it'll be the people from extremely poor countries outside the EU - because those are the people who most desperately need to help their families etc. back home. So leaving the EU isn't likely to make a massive difference in that regard. Since that money is on the whole going to people who need it far more than almost anyone in the UK, and the actual amounts, as a proportion of our GDP, are tiny, I don't see at as being something you could reasonably object to.
Incidentally, migrants sending money back home amounts to an indirect form of aid to poorer countries - and I have heard it suggested that it's a much more effective form of aid than simply giving money to foreign Governments (I don't know whether that's true or not). There is perhaps a case for saying that should therefore be included in the figures for the amount of aid we give, but that's a completely separate argument.