None of us choose the circumstances of our birth. We don't choose where we are born, just like we don't choose our sex, our sexuality, or our skin/hair/eye colour. So to discriminate on birth location is just as immoral as sexist, homophobic or racist discrimination. We all get one shot at life, so to force someone to remain in the same place for their entire life against their will ruins their one shot.
Immigration has been good for the economy. Immigrants fill job vacancies, which creates economic growth and therefore creates more jobs for everyone. Immigrants pay more in taxes than they use in public services. There would be no economy if there were no people, and the more people there are participating in the economy, the stronger the economy will be. The richest ocuntries in the world are the countries that have experienced the most migration. That's no coincidence.
Immigrants tend to be motivated young adults. Our universities are dependent on the fees of foreign-born students. The non-migrant UK population on the other hand is ageing. We are all living longer, and the baby boomer generation are approaching retirement. Without young, economically active immigrants, there will be no-one to fund the ever-growing bill for pensions and the healthcare pensioners rely on. The need to pay for social care for the elderly is also rising. These services rely on migrant labour.