Part of that will be to do with not knowing any of the language, English is more universal and America probably a bit hard to get to. They’re certainly not coming here for a warm embrace
As for numbers, We won’t be overrun. Quite the opposite - globally the population is set to decline in many major countries from around 2060 because people are getting older and are having less and less babies, women are getting more access to education and aren’t staying at home to be baby factories. Populations in Spain, India, Japan, China, Portugal, South Korea (tots of 40 countries) are set to decline by as much as 50%
The birth rate in England and Wales is now the lowest it has ever been since records began. Only 11 babies were born for every 1,000 people in 2018, with 657,076 born overall. To put this in perspective, 957,782 babies were born in 1920 despite the overall population being 22m fewer than it is today.
Brexit will see reduced EU immigration, our birth rates decline meaning the only way to continue to grow the population will be immigration. We’ll have an ageing population, and people right now will be reluctant to have kids with job insecurity and COVID making it harder for new couples to form.