Was immigration a source for good when the Goths and Vandals entered the bounds of the Roman Empire in vast swathes, completely failed to integrate and subsequently ravaged it? If you like we could go even further back to the time of the ancient Hebrews, and ask the Caananites and the inhabitants of Jericho what they thought about incomers.
Mass, unintegrated immigration invariably results in the destruction of distinct indigenous culture. The Europeans did it in the Americas and the Antipodes and we can see the Han Chinese doing it today in Xinjiang, Inner Mongolia and, of course, Tibet. People with unique common histories and value systems tend to band together into those great bastions of diversity, nation states. Attempts to force distinct groups together into artificially fashioned, heterogenous structures like the Ottoman Empire, USSR, Czechoslovakia and Yugoslavia without any kind of democratic mandate inevitably result in failure. Yes, immigrants as a collective group contribute more in tax to the Kingdom than they cost us in handouts, but as the author probably well knows this is because a handful of wealthy business magnates and financiers (mostly Americans) and millionaire football club managers and celebrities (Madonna, et al) heavily distort the figures. The vast bulk of the immigrant population is, unfortunately, a drain of social security; and additionally it puts excessive strain on public services (NHS, transport etc), contributes to congestion and drives house prices up and wages down (the blame for this lies largely with greedy and exploitative employers, of course).
Are these "benefits" worth the cost to democracy and social solidarity?