Immigrants work in the UK, pay their taxes and then return to their own countries before they burden the UK for services that those taxes pay for - benefits, pension, NHS etc.
You have a citation for that? Because I saw a study that said, with much fanfare, that they're on average net contributors. And then another that controlled for age and concluded that's because they're more concentrated around the peak age for working; compare like for like and there's not much difference.
While you may think that supports your point, it doesn't. We don't know that they go home when they're old, because they haven't been here long enough to become old.