Bullshit, the economy does not collapse without immigration. That has never happened anywhere. Wages go up and unemployment drops with vacancies, which is beneficial to current citizens of a country. A Migrant coming in and working for a month then taking all of his pay back to his home does not help the British economy in any way. In fact that month or three harms Britain, because people would work summers can't compete to get the jobs (College and High school students, recent grads unsure of a career choice, etc..). Those people would have turned around and invested the money back into the UK.
A healthy economy means increased birthrates, so no need to import people for the low skilled jobs. The low birth rate issue is a common one with all of the first world countries currently. Immigration is not the the only, or necessarily best, answer.
You can't grasp basic common sense economics, and yet you claim others of providing stupid easy answers.