Only one group of people has the unrestricted right to immigrate wherever they choose: refugees, as defined by the UN Convention to that effect. These folk are often from developing countries with politically unstable regimes: Syria, Colombia, and the Democratic Republic of the Congo, among others. Any further discussion of immigration should be constrained to purely socioeconomic effects on the host country, and that debate is academic.
The academic evidence runs contrary to journalists' simplistic views. Sir Paul Collier, in his recent work Exodus, reviews the evidence to date, asking how much migration is beneficial for an economy. To a large extent, migration is good, allowing new ideas and variegated skills to enter a nation. For example, large waves of postbellum US immigration yielded men like Andrew Carnegie and Nikola Tesla. However, as cultural diversity increases, trust, social cohesion, and mutual regard decline - leading to lower economic growth. The right questions for policymakers, then, are: (1) how much diversity is required; and (2) how rapidly are migrants integrated into host societies? The evidence requires us to answer difficult questions.
In the case of the UK, another question is that of land. Britain is not Canada, with large swathes of forest. The UK is already comfortably crowded with 63 million people, and making room for more is difficult. Destroying the Green Belt is both foolish and environmentally unsound, so sensible immigration restrictions are key.
British voters, in electing UKIP, are sending a clear message to policymakers to enact such restrictions. I do not know precisely what a good policy would look like, but I can offer one suggestion: other than refugees, only immigrants that offer good skill complementarity should be admitted to Britain. This does not just mean high-skilled labourers, of course, but rather those who can meaningfully contribute to the economy. On the other hand, if too much cultural diversity is what we are concerned with, it makes sense to admit Commonwealth migrants: those from Jamaica, Kenya, Canada, etc., who have already adopted British customs. If the UK does not enact these restrictions, then its economic and cultural future is at peril.