We have a desperately low birth rate. We desperately need young immigrants who are, almost by definition, ambitious. You don't risk life and limb to eg claim asylum here unless you have to and want to better yourself. They are cheap too. We haven't had to pay for their hospital births or their education. If they subsequently marry or bring their wives and children here that's an added bonus.
The problem we have is not immigrants, it's the concentration of wealth increasingly in the hands of the super rich. The richest 50 families in this country own as much as the poorest 50% of the population. We need to tax more fairly. Stop allowing the super rich to off shore their profits etc etc. We need to tax all our citizens fairly. I believe Rishi Sunk paid only 25% tax on his enormous wealth.
Many countries can afford to pay old age pensions at a higher rate than us (we have the lowest in Europe I believe), despite being one of the 5/6th richest countries in the world. Many countries pay OAPs at a younger age than us despite having much lower per capital incomes eg China - most women get their OAP at 55 (only managerial women wait to 60! ) It's the poor who do not get a pension at all if we keep raising the pension age as they die much younger than the rich.
It's political will that we need. Stop privatising our wealth and services, Water, Electricity, Rail, NHS etc etc and allowing the profits to be sent abroad or into the pockets of our super rich leaving us with their ill gotten debt.