Unless you want to Balkanize yourself immigration is probably not the answer.

The US is also probably not the best example because it’s a very different situation. Even in the 19th and 18th centuries natural birth rates accounted for about half of the population growth in the US.

The current projection for the UK is that between 2021 and 2036 immigration will account for 92% of the population growth, and based on the previous 2-3 years this might actually be an underestimate.